tcp: move tcp_rack_update_reo_wnd() to tcp_input.c

tcp: move tcp_rack_update_reo_wnd() to tcp_input.c

tcp_rack_update_reo_wnd() is called only once from tcp_ack()

Move it to tcp_input.c so that it can be inlined by the compiler
to save space and cpu cycles.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/2 grow/shrink: 1/0 up/down: 110/-153 (-43)
Function                                     old     new   delta
tcp_ack                                     5631    5741    +110
__pfx_tcp_rack_update_reo_wnd                 16       -     -16
tcp_rack_update_reo_wnd                      137       -    -137
Total: Before=22572723, After=22572680, chg -0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260127032147.3498272-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>