staging/lustre: Remove ns_is_client()

Since staging tree code is just the client, ns_is_client is always
true, so change all callers as such and drop all the
dead code for when it's false.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>