net: Make sock_splice_read() use copy_splice_read() by default

Make sock_splice_read() use copy_splice_read() by default as
file_splice_read() will return immediately with 0 as a socket has no
pagecache and is a zero-size file.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Jens Axboe <axboe@kernel.dk>
cc: netdev@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/20230522135018.2742245-14-dhowells@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

net/socket.c

@@ -1093,7 +1093,7 @@ 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 generic_file_splice_read(file, ppos, pipe, len, flags);
+		return copy_splice_read(file, ppos, pipe, len, flags);
 
 	return sock->ops->splice_read(sock, ppos, pipe, len, flags);
 }