tcp: move tcp_stream_memory_free() to tcp.c

tcp: move tcp_stream_memory_free() to tcp.c

Moving tcp_stream_memory_free() to tcp.c allows the compiler
to (auto)inline it from tcp_poll() and tcp_sendmsg_locked()
for better performance.

$ scripts/bloat-o-meter -t vmlinux.old vmlinux.new
add/remove: 0/0 grow/shrink: 2/0 up/down: 118/0 (118)
Function                                     old     new   delta
tcp_poll                                     840     923     +83
tcp_sendmsg_locked                          4217    4252     +35
Total: Before=22573095, After=22573213, chg +0.00%

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Link: https://patch.msgid.link/20260122090228.1678207-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>