tcp: move tcp_rate_skb_sent() to tcp_output.c

tcp: move tcp_rate_skb_sent() to tcp_output.c

It is only called from __tcp_transmit_skb() and __tcp_retransmit_skb().

Move it in tcp_output.c and make it static.

clang compiler is now able to inline it from __tcp_transmit_skb().

gcc compiler inlines it in the two callers, which is also fine.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Link: https://patch.msgid.link/20260114165109.1747722-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>