ovl: add NULL check in ovl_alloc_inode

This was detected by fault injection test
Signed-off-by: Hirofumi Nakagawa <nklabs@gmail.com>
Fixes: 13cf199d ("ovl: allocate an ovl_inode struct")
Cc: <stable@vger.kernel.org> # v4.13

fs/overlayfs/super.c

@@ -174,6 +174,9 @@ static struct inode *ovl_alloc_inode(struct super_block *sb)
 {
 	struct ovl_inode *oi = kmem_cache_alloc(ovl_inode_cachep, GFP_KERNEL);
 
+	if (!oi)
+		return NULL;
+
 	oi->cache = NULL;
 	oi->redirect = NULL;
 	oi->version = 0;