ovl: Pass ovl_get_nlink() parameters in right order

Right now we seem to be passing index as "lowerdentry" and origin.dentry
as "upperdentry". IIUC, we should pass these parameters in reversed order
and this looks like a bug.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Amir Goldstein <amir73il@gmail.com>
Fixes: caf70cb2 ("ovl: cleanup orphan index entries")
Cc: <stable@vger.kernel.org> #v4.13
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>