NFS: Don't force unnecessary cache invalidation in nfs_update_inode()

If we managed to revalidate all the attributes, then there is no reason
to mark them as invalid again. We do, however want to ensure that we
set nfsi->attrtimeo correctly.

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