tcp: rename sk_wmem_free_skb

sk_wmem_free_skb() is only used by TCP.

Rename it to make this clear, and move its declaration to
include/net/tcp.h

Signed-off-by: Talal Ahmad <talalahmad@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Acked-by: Arjun Roy <arjunroy@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>