socket: do a generic_file_splice_read when proto_ops has no splice_read

splice(2) fails with -EINVAL when called reading on a socket with no splice_read
set in its proto_ops (such as vsock sockets). Switch this to fallbacks to a
generic_file_splice_read instead.
Signed-off-by: Slavomir Kaslev <kaslevs@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/socket.c

@@ -853,7 +853,7 @@ 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 generic_file_splice_read(file, ppos, pipe, len, flags);
 
 	return sock->ops->splice_read(sock, ppos, pipe, len, flags);
 }