ovl: decode pure lower dir file handles

Similar to decoding a pure upper dir file handle, decoding a pure lower
dir file handle is implemented by looking an overlay dentry of the same
path as the pure lower path and verifying that the overlay dentry's
real lower matches the decoded real lower file handle.

Unlike the case of upper dir file handle, the lookup of overlay path by
lower real path can fail or find a mismatched overlay dentry if any of
the lower parents have been copied up and renamed. To address this case
we will need to check if any of the lower parents are indexed.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>