sysv_sem: semusz depends on semume.

Size of the per-process semaphore undo structure (semusz) depends on
the number of the per-process undos. If kern.ipc.semume is adjusted,
semusz must be adjusted as well, and it makes no sense to delegate
adjustment to user. Make it automatic.

Reported and tested by: Olef <o.vandestadt@gmail.com>
PR: 250361
Reviewed by: jhb, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D26826