ctl(4): Do hole-punching for UNMAP to file-backed LUNs

This adds support for SCSI UNMAP command to file-backed LUNs, if the
underlying file system has a non-zerofilling VOP_DEALLOCATE
implementation where some or all parts of the requested operation range
may be deallocated.

Sponsored by: The FreeBSD Foundation
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D31922