ovl: whiteout inode sharing

Share inode with different whiteout files for saving inode and speeding up
delete operation.

If EMLINK is encountered when linking a shared whiteout, create a new one.
In case of any other error, disable sharing for this super block.

Note: ofs->whiteout is protected by inode lock on workdir.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c
index 36b6078..01755bc 100644
--- a/fs/overlayfs/util.c
+++ b/fs/overlayfs/util.c
@@ -707,7 +707,8 @@ static void ovl_cleanup_index(struct dentry *dentry)
 		index = NULL;
 	} else if (ovl_index_all(dentry->d_sb)) {
 		/* Whiteout orphan index to block future open by handle */
-		err = ovl_cleanup_and_whiteout(indexdir, dir, index);
+		err = ovl_cleanup_and_whiteout(OVL_FS(dentry->d_sb),
+					       dir, index);
 	} else {
 		/* Cleanup orphan index entries */
 		err = ovl_cleanup(dir, index);