Add to CTL support for logical block provisioning threshold notifications.

For ZVOL-backed LUNs this allows to inform initiators if storage's used or
available spaces get above/below the configured thresholds.

MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.
Ticket:		#6468

(cherry picked from commit e22f45febced94fa20499d8f0e1ac165420b311f)