NFS: Fix dentry revalidation for NFSv4 referrals and mountpoint crossings

As long as the directory contents haven't changed, we should just let the
path walk proceed to cross the mountpoint. Apart from being an optimisation
in the case of 'nohide' mountpoint traversals, it also fixes an issue with
referrals: referral inodes don't have valid filehandles, so calling
nfs_revalidate_inode() on them is a bug.

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