mountd: Promote missing netmask log to LOG_ERR
Historical network classes are deprecated, but currently still supported
in mountd with a LOG_WARNING-priority message added by Mike Karels in
- Upgrade the log to LOG_ERR well in advance of this becoming a
fatal error.
Reviewed by: rmacklem
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48795