Reject F_SETLK_REMOTE commands when sysid == 0.
A sysid of 0 denotes the local system, and some handlers for remote
locking commands do not attempt to deal with local locks. Note that
F_SETLK_REMOTE is only available to privileged users as it is intended
to be used as a testing interface.
Reviewed by: kib
Reported by: email@example.com
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D19702