tcp: inline __tcp_v4_send_check()

Inline __tcp_v4_send_check(), like __tcp_v6_send_check().

Move tcp_v4_send_check() to tcp_output.c close to
its fast path caller (__tcp_transmit_skb()).

Note __tcp_v4_send_check() is still out-of-line for tcp4_gso_segment()
because it is called in an unlikely() section.

$ scripts/bloat-o-meter -t vmlinux.0 vmlinux.1
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-9 (-9)
Function                                     old     new   delta
__tcp_v4_send_check                          130     121      -9
Total: Before=25143100, After=25143091, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260223100729.3761597-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>