tcp: inline tcp_filter()

tcp: inline tcp_filter()

This helper is already (auto)inlined from IPv4 TCP stack.

Make it an inline function to benefit IPv6 as well.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/2 grow/shrink: 1/0 up/down: 30/-49 (-19)
Function                                     old     new   delta
tcp_v6_rcv                                  3448    3478     +30
__pfx_tcp_filter                              16       -     -16
tcp_filter                                    33       -     -33
Total: Before=24891904, After=24891885, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260205164329.3401481-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>