tcp: change tcp_filter() to return the reason by value

tcp: change tcp_filter() to return the reason by value

sk_filter_trim_cap() will soon return the reason by value,
do the same for tcp_filter().

Note:

tcp_filter() is no longer inlined. Following patch will inline it again.

$ scripts/bloat-o-meter -t vmlinux.4 vmlinux.5
add/remove: 2/0 grow/shrink: 0/2 up/down: 186/-43 (143)
Function                                     old     new   delta
tcp_filter                                     -     154    +154
__pfx_tcp_filter                               -      32     +32
tcp_v4_rcv                                  3152    3143      -9
tcp_v6_rcv                                  3169    3135     -34
Total: Before=29722640, After=29722783, chg +0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260409145625.2306224-5-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>