Implement range checks between UNMAP and READ/WRITE commands.

Before this change UNMAP completely blocked other I/Os while running.
Now it blocks only colliding ones, slowing down others only due to ZFS
locks collisions.

Sponsored by:	iXsystems, Inc.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271503 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f