NFS: Fix a jiffie wraparound issue

dentry verifiers are always set to the parent directory's
cache_change_attribute. There is no reason to be testing for anything other
than equality when we're trying to find out if the dentry has been checked
since the last time the directory was modified.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>