Consistently use sizeof for the known buffer when calling memcpy/memset

This fixes a copy-pasto when handling LEAF_usmUserAuthKeyChange where
SNMP_AUTH_KEY_SIZ was used instead of SNMP_PRIV_KEY_SIZ, resulting in a
potential buffer overflow scenario

MFC after: 2 weeks
CID: 1008326, 1009675
Reported by: Coverity
Sponsored by: EMC / Isilon Storage Division