NFSd: use network-namespace-aware cache registering routines

v2: cache_register_net() and cache_unregister_net() GPL exports added

This is a cleanup patch. Hope, some day generic cache_register() and
cache_unregister() will be removed.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>