tcp: make tcp_v{4,6}_send_check() static

tcp_v{4,6}_send_check() are only called from tcp_output.c
and should be made static so that the compiler does not need
to put an out of line copy of them.

Remove (struct inet_connection_sock_af_ops) send_check field
and use instead @net_header_len.

Move @net_header_len close to @queue_xmit for data locality
as both are used in TCP tx fast path.

$ scripts/bloat-o-meter -t vmlinux.2 vmlinux.3
add/remove: 0/2 grow/shrink: 0/3 up/down: 0/-172 (-172)
Function                                     old     new   delta
__tcp_transmit_skb                          3426    3423      -3
tcp_v4_send_check                            136     132      -4
mptcp_subflow_init                           777     763     -14
__pfx_tcp_v6_send_check                       16       -     -16
tcp_v6_send_check                            135       -    -135
Total: Before=25143196, After=25143024, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260223100729.3761597-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>