NFSd: remove hard-coded dereferences to name-to-id and id-to-name caches

These dereferences to global static caches are redundant. They also prevents
converting these caches into per-net ones. So this patch is cleanup + precursor
of patch set,a which will make them per-net.

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