To obey NFS cache semantics, the client must verify the cached
attributes when a file is opened.  In most cases this is done by a call to
d_validate as one of the last steps in path_walk.

However for the root of a filesystem, d_validate is only ever called
on the mounted-on filesystem (except when the path ends '.' or '..').
So NFS has no chance to validate the attributes.

So, in nfs_opendir, we revalidate the attributes if the opened
directory is the mountpoint.  This may cause double-validation for "."
and ".." lookups, but that is better than missing regular /path/name
lookups completely.
Signed-off-by: NNeilBrown <neilb@suse.de>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>