tcp: return a drop_reason from tcp_add_backlog()

tcp: return a drop_reason from tcp_add_backlog()

Part of a stack canary removal from tcp_v{4,6}_rcv().

Return a drop_reason instead of a boolean, so that we no longer
have to pass the address of a local variable.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-37 (-37)
Function                                     old     new   delta
tcp_v6_rcv                                  3133    3129      -4
tcp_v4_rcv                                  3206    3202      -4
tcp_add_backlog                             1281    1252     -29
Total: Before=25567186, After=25567149, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260409101147.1642967-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>