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: 018584697533 ("netfs: Add a function to extract an iterator into a 
scatterlist")
Reported-by: kernel test robot <l...@intel.com>
Reported-by: Dan Carpenter <erro...@gmail.com>
Signed-off-by: David Howells <dhowe...@redhat.com>
cc: Steve French <sfre...@samba.org>
cc: Jeff Layton <jlay...@kernel.org>
cc: linux-c...@vger.kernel.org
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/oe-kbuild-all/202302261115.p3tqi1zo-...@intel.com/
---
 fs/netfs/iterator.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/netfs/iterator.c b/fs/netfs/iterator.c
index f00d43b8ac0a..e9a45dea748a 100644
--- a/fs/netfs/iterator.c
+++ b/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);
 
--
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