tcp: use __skb_push() in __tcp_transmit_skb()

tcp: use __skb_push() in __tcp_transmit_skb()

We trust MAX_TCP_HEADER to be large enough.

Using the inlined version of skb_push() trades 8 bytes
of text for better performance of TCP TX fast path.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 1/0 up/down: 8/0 (8)
Function                                     old     new   delta
__tcp_transmit_skb                          3181    3189      +8
Total: Before=24896035, After=24896043, chg +0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260203044226.3489941-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>