rpc.lockd: Explicitly cast enum nlm_stats values to enum nlm4_stats.

NLM 4 status values are a superset of the older NLM protocol so these
casts are safe while pacifying -Wenum-conversion warnings from GCC.

Reviewed by: rmacklem
Differential Revision: https://reviews.freebsd.org/D36816

(cherry picked from commit 3b887005b4bb215e0e052ae81fe31c2c0d5f4f35)