Unionfs: fixes to cache-coherency checking code

Unionfs: fixes to cache-coherency checking code

Under memory pressure, in_newer_lower could be called on uninitialized
dentries or inodes.  So return 0 safely in that case (rather than oops).
This is OK because is_newer_lower will be called again on the same object(s)
and cache-coherency will be validated and maintained then.

Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>