NFS: use SUNRPC cache creation and destruction helper for DNS cache

This cache was the first containerized and doesn't use net-aware cache
creation and destruction helpers.
This is a cleanup patch which just makes code looks clearer and reduce amount
of lines of code.

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