selftests: fib-onlink: Add a test case for IPv4 multicast gateway

selftests: fib-onlink: Add a test case for IPv4 multicast gateway

A multicast gateway address should be rejected when "onlink" is
specified, but it is only tested as part of the IPv6 tests. Add an
equivalent IPv4 test.

 # ./fib-onlink-tests.sh -v
 [...]
 COMMAND: ip ro add table 254 169.254.101.12/32 via 233.252.0.1 dev veth1 onlink
 Error: Nexthop has invalid gateway.

 TEST: Invalid gw - multicast address                      [ OK ]
 [...]
 COMMAND: ip ro add table 1101 169.254.102.12/32 via 233.252.0.1 dev veth5 onlink
 Error: Nexthop has invalid gateway.

 TEST: Invalid gw - multicast address, VRF                 [ OK ]
 [...]
 Tests passed:  37
 Tests failed:   0

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260111120813.159799-4-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>