tcp: move tcp_rate_check_app_limited() to tcp.c

tcp: move tcp_rate_check_app_limited() to tcp.c

tcp_rate_check_app_limited() is used from tcp_sendmsg_locked()
fast path and from other callers.

Move it to tcp.c so that it can be inlined in tcp_sendmsg_locked().

Small increase of code, for better TCP performance.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 1/0 up/down: 87/0 (87)
Function                                     old     new   delta
tcp_sendmsg_locked                          4217    4304     +87
Total: Before=22566462, After=22566549, chg +0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Link: https://patch.msgid.link/20260121095923.3134639-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>