NFS: Simplify the cache invalidation code

Now that NFSv2 and NFSv3 have simulated change attributes,
instead of using all three of mtime, ctime and change attribute to
manage data cache consistency, we can simplify the code to just use
the change attribute.

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