ovl_indexdir_cleanup() is called on mount of overayfs with nfs_export
feature to cleanup stale index records for lower and upper files that have
been deleted while overlayfs was offline.

This has the side effect (good or bad) of pre populating inode cache with
all the copied up upper inodes, while verifying the index entries.

For copied up directories, the upper file handles are decoded to conncted
upper dentries.  This has the even bigger side effect of reading the
content of all the parent upper directories which may take significantly
more time and IO than just reading the upper inodes.

Do not request connceted upper dentries for verifying upper directory index
entries, because we have no use for the connected dentry.
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>