nfs: fix another case of NULL/IS_ERR confusion wrt folio pointers

Dan has been improving on the smatch error pointer checks, and pointed
at another case where the __filemap_get_folio() conversion to error
pointers had been overlooked.  This time because it was hidden behind
the filemap_grab_folio() helper function that is a wrapper around it.
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Cc: Anna Schumaker <anna@kernel.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nfs/dir.c

@@ -402,7 +402,7 @@ static struct folio *nfs_readdir_folio_get_locked(struct address_space *mapping,
 	struct folio *folio;
 
 	folio = filemap_grab_folio(mapping, index);
-	if (!folio)
+	if (IS_ERR(folio))
 		return NULL;
 	nfs_readdir_folio_init_and_validate(folio, cookie, change_attr);
 	return folio;