NFS: Fix a resolution problem with nfs_inode->cache_change_attribute

The cache_change_attribute is used to decide whether or not a directory has
changed, in which case we may need to look it up again. Again, the use of
'jiffies' leads to an issue of resolution.

Once again, the fix is to change nfs_inode->cache_change_attribute, and
just make it a simple counter.

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