NFS: Don't redirty the attribute cache in nfs_wcc_update_inode()

If we received weak cache consistency data from the server, then those
attributes are up to date, and there is no reason to mark them as
dirty in the attribute cache.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>