In addition to r275481 allow threshold notifications work without UNMAP.

While without UNMAP support there is not much initiator can do about it,
the administrator still better be notified about the storage overflow.

MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275512 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f