Reduce code duplication in rpc.lockd.

Reuse create_service code instead of duplicating it in
lookup_addresses for kernel NLM.

As a (good) side effect this also fixed a few issues that were
already fixed in the former but never applied to the latter.

Reviewed by: kevlo
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D11259