NFS: Fix dcache revalidation bugs

We don't need to force a dentry lookup just because we're making changes to
the directory.

Don't update nfsi->cache_change_attribute in nfs_end_data_update: that
overrides the NFSv3/v4 weak consistency checking that tells us our update
was the only one, and that tells us the dcache is still valid.

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