Merge branch 'udp-retire-udp-lite'

Kuniyuki Iwashima says:

====================
udp: Retire UDP-Lite.

In 2023, syzbot found a null-ptr-deref bug triggered when UDP-Lite
attempted to charge an skb after the total memory usage for UDP-Lite
_and_ UDP exceeded a system-wide threshold, net.ipv4.udp_mem[1].

Since this threshold is shared with UDP, the bug would have been
easy to trigger if any real-world applications were using UDP-Lite;
however, only syzbot ever stumbled upon it.

The bug had persisted since 2016, suggesting that UDP-Lite had
remained unused for 7 years at that point.

The bug was fixed in commit ad42a35bdfc6 ("udplite: Fix NULL pointer
dereference in __sk_mem_raise_allocated()."), and we added another
commit be28c14ac8bb ("udplite: Print deprecation notice.") to
announce the deprecation plan.

Since then, no one has complained, so it is time to officially
retire UDP-Lite.

This series first removes IPv6 and IPv4 UDP-Lite sockets, then
gradually cleans up the remaining dead/unnecessary code within
the UDP stack.

By removing a bunch of conditionals for UDP-Lite from the fast
path, udp_rr with 20,000 flows sees a 10% increase in pps
(13.3 Mpps -> 14.7 Mpps)  on an AMD EPYC 7B12 (Zen 2) 64-Core
Processor platform.

[ With FDO, the baseline is much higher and the delta was ~3%,
  20.1 Mpps -> 20.7 Mpps ]

Before:

$ nstat > /dev/null; sleep 1; nstat | grep Udp
Udp6InDatagrams                 14013408           0.0
Udp6OutDatagrams                14013128           0.0

After:

$ nstat > /dev/null; sleep 1; nstat | grep Udp
Udp6InDatagrams                 15491971           0.0
Udp6OutDatagrams                15491671           0.0

$ ./scripts/bloat-o-meter vmlinux.before vmlinux.after
add/remove: 13/75 grow/shrink: 11/75 up/down: 13777/-18401 (-4624)
Function                                     old     new   delta
udp4_gro_receive                             872     866      -6
udp6_gro_receive                             910     903      -7
udp_rcv                                       32    1727   +1695
udpv6_rcv                                     32    1450   +1418
__udp4_lib_rcv                              2045       -   -2045
__udp6_lib_rcv                              2084       -   -2084
udp_unicast_rcv_skb                          160     149     -11
udp6_unicast_rcv_skb                         196     181     -15
__udp4_lib_mcast_deliver                     925     846     -79
__udp6_lib_mcast_deliver                     922     810    -112
__udp4_lib_lookup                            973     969      -4
__udp6_lib_lookup                            940     929     -11
__udp4_lib_lookup_skb                        106     100      -6
__udp6_lib_lookup_skb                         71      66      -5
udp4_lib_lookup_skb                          132     127      -5
udp6_lib_lookup_skb                           87      81      -6
udp_queue_rcv_skb                            326     356     +30
udpv6_queue_rcv_skb                          331     361     +30
udp_queue_rcv_one_skb                       1233     914    -319
udpv6_queue_rcv_one_skb                     1250     930    -320
__udp_enqueue_schedule_skb                  1067     995     -72
udp_rcv_segment                              520     480     -40
udp_post_segment_fix_csum                    120       -    -120
udp_lib_checksum_complete                    200      84    -116
udp_err                                       27    1103   +1076
udpv6_err                                     36    1417   +1381
__udp4_lib_err                              1112       -   -1112
__udp6_lib_err                              1448       -   -1448
udp_recvmsg                                 1149     994    -155
udpv6_recvmsg                               1349    1294     -55
udp_sendmsg                                 2730    2648     -82
udp_send_skb                                 909     681    -228
udpv6_sendmsg                               3022    2861    -161
udp_v6_send_skb                             1214     952    -262
...
Total: Before=18446744073748075501, After=18446744073748070877, chg -0.00%
====================

Link: https://patch.msgid.link/20260311052020.1213705-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>