ceph: Fix error handling in fill_readdir_cache()

__filemap_get_folio() returns an ERR_PTR, not NULL.  There are extensive
assumptions that ctl->folio is NULL, not an error pointer, so it seems
better to fix this one place rather than change all the places which
check ctl->folio.

Fixes: baff974 ("ceph: Convert ceph_readdir_cache_control to store a folio")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Link: https://lore.kernel.org/r/20250304154818.250757-1-willy@infradead.org
Cc: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Matthew Wilcox (Oracle)

fs/ceph/inode.c

@@ -1870,9 +1870,12 @@ static int fill_readdir_cache(struct inode *dir, struct dentry *dn,
 
 		ctl->folio = __filemap_get_folio(&dir->i_data, pgoff,
 				fgf, mapping_gfp_mask(&dir->i_data));
-		if (!ctl->folio) {
+		if (IS_ERR(ctl->folio)) {
+			int err = PTR_ERR(ctl->folio);
+
+			ctl->folio = NULL;
 			ctl->index = -1;
-			return idx == 0 ? -ENOMEM : 0;
+			return idx == 0 ? err : 0;
 		}
 		/* reading/filling the cache are serialized by
 		 * i_rwsem, no need to use folio lock */