[NET]: Make sure sockets implement splice_read

Fixes a segmentation fault when trying to splice from a non-TCP socket.
Signed-off-by: NRémi Denis-Courmont <rdenis@simphalempin.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/socket.c

@@ -701,6 +701,9 @@ static ssize_t sock_splice_read(struct file *file, loff_t *ppos,
 {
 	struct socket *sock = file->private_data;
 
+	if (unlikely(!sock->ops->splice_read))
+		return -EINVAL;
+
 	return sock->ops->splice_read(sock, ppos, pipe, len, flags);
 }