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>

net/ipv4/tcp.c

@@ -1229,13 +1229,12 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 	flags = msg->msg_flags;
 
 	if ((flags & MSG_ZEROCOPY) && size) {
-		skb = tcp_write_queue_tail(sk);
-
 		if (msg->msg_ubuf) {
 			uarg = msg->msg_ubuf;
 			net_zcopy_get(uarg);
 			zc = sk->sk_route_caps & NETIF_F_SG;
 		} else if (sock_flag(sk, SOCK_ZEROCOPY)) {
+			skb = tcp_write_queue_tail(sk);
 			uarg = msg_zerocopy_realloc(sk, size, skb_zcopy(skb));
 			if (!uarg) {
 				err = -ENOBUFS;