tcp: move tcp_v6_send_check() to tcp_output.c

Move tcp_v6_send_check() so that __tcp_transmit_skb() can inline it.

$ scripts/bloat-o-meter -t vmlinux.1 vmlinux.2
add/remove: 0/0 grow/shrink: 1/0 up/down: 105/0 (105)
Function                                     old     new   delta
__tcp_transmit_skb                          3321    3426    +105
Total: Before=25143091, After=25143196, chg +0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260223100729.3761597-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>