net/tcp: don't peek at tail for io_uring zc

Move tcp_write_queue_tail() to SOCK_ZEROCOPY specific flag as zerocopy
setup for msghdr->ubuf_info doesn't need to peek into the last request.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>