selftests: net: Add tests for ARP probe and DAD NS handling

selftests: net: Add tests for ARP probe and DAD NS handling

Add test cases to verify that ARP probes and DAD Neighbor Solicitations
are handled correctly by the bridge neighbor suppression feature.

When neighbor suppression is enabled on a bridge VXLAN port, the bridge
should reply to ARP/NS messages on behalf of remote hosts when both FDB
and neighbor entries exist, and the answer is known. However, when
either the FDB or the neighbor exists, ARP probes / DAD NS should be
treated like regular ARP requests / NS and flood to VXLAN.

Add two new test functions:

neigh_suppress_arp_probe(): Tests ARP probe handling by triggering
duplicate address detection using arping -D. Verifies that probes are
flooded when the bridge doesn't know the answer, and suppressed when FDB
and neighbor entries exist.

neigh_suppress_dad_ns(): Tests DAD NS handling by constructing DAD NS
packets using mausezahn and verifies correct flooding/suppression
behavior.

Before the previous patch:

$ ./test_bridge_neigh_suppress.sh -t "neigh_suppress_arp_probe neigh_suppress_dad_ns"

Per-port ARP probe suppression
------------------------------
TEST: ARP probe suppression                                         [ OK ]
TEST: "neigh_suppress" is on                                        [ OK ]
TEST: ARP probe suppression                                         [FAIL]
TEST: FDB and neighbor entry installation                           [ OK ]
TEST: arping                                                        [FAIL]
TEST: ARP probe suppression                                         [FAIL]
TEST: neighbor removal                                              [ OK ]
TEST: ARP probe suppression                                         [FAIL]
TEST: "neigh_suppress" is off                                       [ OK ]
TEST: ARP probe suppression                                         [FAIL]

Per-port DAD NS suppression
---------------------------
TEST: DAD NS suppression                                            [ OK ]
TEST: "neigh_suppress" is on                                        [ OK ]
TEST: DAD NS suppression                                            [FAIL]
TEST: FDB and neighbor entry installation                           [ OK ]
TEST: DAD NS suppression                                            [FAIL]
TEST: neighbor removal                                              [ OK ]
TEST: DAD NS suppression                                            [FAIL]
TEST: DAD NS proxy NA reply                                         [FAIL]
TEST: "neigh_suppress" is off                                       [ OK ]
TEST: DAD NS suppression                                            [FAIL]

Tests passed:   10
Tests failed:   10

After the previous patch:

$ ./test_bridge_neigh_suppress.sh -t "neigh_suppress_arp_probe neigh_suppress_dad_ns"

Per-port ARP probe suppression
------------------------------
TEST: ARP probe suppression                                         [ OK ]
TEST: "neigh_suppress" is on                                        [ OK ]
TEST: ARP probe suppression                                         [ OK ]
TEST: FDB and neighbor entry installation                           [ OK ]
TEST: arping                                                        [ OK ]
TEST: ARP probe suppression                                         [ OK ]
TEST: neighbor removal                                              [ OK ]
TEST: ARP probe suppression                                         [ OK ]
TEST: "neigh_suppress" is off                                       [ OK ]
TEST: ARP probe suppression                                         [ OK ]

Per-port DAD NS suppression
---------------------------
TEST: DAD NS suppression                                            [ OK ]
TEST: "neigh_suppress" is on                                        [ OK ]
TEST: DAD NS suppression                                            [ OK ]
TEST: FDB and neighbor entry installation                           [ OK ]
TEST: DAD NS suppression                                            [ OK ]
TEST: neighbor removal                                              [ OK ]
TEST: DAD NS suppression                                            [ OK ]
TEST: DAD NS proxy NA reply                                         [ OK ]
TEST: "neigh_suppress" is off                                       [ OK ]
TEST: DAD NS suppression                                            [ OK ]

Tests passed:  20
Tests failed:   0

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260429062405.1386417-3-danieller@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>