ovl: generalize ovl_verify_origin() and helpers

Remove the "origin" language from the functions that handle set, get
and verify of "origin" xattr and pass the xattr name as an argument.

The same helpers are going to be used for NFS export to get, get and
verify the "upper" xattr for directory index entries.

ovl_verify_origin() is now a helper used only to verify non upper
file handle stored in "origin" xattr of upper inode.

The upper root dir file handle is still stored in "origin" xattr on
the index dir for backward compatibility. This is going to be changed
by the patch that adds directory index entries support.

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