iov: Fix netfs_extract_user_to_sg()

Fix the loop check in netfs_extract_user_to_sg() for extraction from
user-backed iterators to do the body if npages > 0, not if npages < 0
(which it can never be).

This isn't currently used by cifs, which only ever extracts data from BVEC,
KVEC and XARRAY iterators at this level, user-backed iterators having being
decanted into BVEC iterators at a higher level to accommodate the work
being done in a kernel thread.

Found by smatch:
	fs/netfs/iterator.c:139 netfs_extract_user_to_sg() warn: unsigned 'npages' is never less than zero.

Fixes: 0185846 ("netfs: Add a function to extract an iterator into a scatterlist")
Reported-by: kernel test robot <[email protected]>
Link: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Reported-by: Dan Carpenter <[email protected]>
Link: https://lore.kernel.org/r/Y/yYnAhoAYDBKixX@kili
Reviewed-by: Paulo Alcantara (SUSE) <[email protected]>
Signed-off-by: David Howells <[email protected]>
cc: Jeff Layton <[email protected]>
cc: [email protected]
cc: [email protected]
Signed-off-by: Steve French <[email protected]>

Author: dhowells

fs/netfs/iterator.c

@@ -134,7 +134,7 @@ static ssize_t netfs_extract_user_to_sg(struct iov_iter *iter,
 		npages = DIV_ROUND_UP(off + len, PAGE_SIZE);
 		sg_max -= npages;
 
-		for (; npages < 0; npages--) {
+		for (; npages > 0; npages--) {
 			struct page *page = *pages;
 			size_t seg = min_t(size_t, PAGE_SIZE - off, len);