Fix a potential infinite loop in extract_user_to_sg()

Fix extract_user_to_sg() so that it will break out of the loop if
iov_iter_extract_pages() returns 0 rather than looping around forever.

[Note that I've included two fixes lines as the function got moved to a
different file and renamed]

Fixes: 85dd2c8 ("netfs: Add a function to extract a UBUF or IOVEC into a BVEC iterator")
Fixes: f5f82cd ("Move netfs_extract_iter_to_sg() to lib/scatterlist.c")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Steve French <sfrench@samba.org>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: netfs@lists.linux.dev
Link: https://lore.kernel.org/r/1967121.1714034372@warthog.procyon.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: dhowells

lib/scatterlist.c

@@ -1124,7 +1124,7 @@ static ssize_t extract_user_to_sg(struct iov_iter *iter,
 	do {
 		res = iov_iter_extract_pages(iter, &pages, maxsize, sg_max,
 					     extraction_flags, &off);
-		if (res < 0)
+		if (res <= 0)
 			goto failed;
 
 		len = res;