netfs: Fix a NULL vs IS_ERR() check in netfs_perform_write()

The netfs_grab_folio_for_write() function doesn't return NULL, it returns
error pointers.  Update the check accordingly.

Fixes: c38f4e9 ("netfs: Provide func to copy data to pagecache for buffered write")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/29fb1310-8e2d-47ba-b68d-40354eb7b896@moroto.mountain/

fs/netfs/buffered_write.c

@@ -221,10 +221,11 @@ ssize_t netfs_perform_write(struct kiocb *iocb, struct iov_iter *iter,
 		if (unlikely(fault_in_iov_iter_readable(iter, part) == part))
 			break;
 
-		ret = -ENOMEM;
 		folio = netfs_grab_folio_for_write(mapping, pos, part);
-		if (!folio)
+		if (IS_ERR(folio)) {
+			ret = PTR_ERR(folio);
 			break;
+		}
 
 		flen = folio_size(folio);
 		offset = pos & (flen - 1);