MFV (partial) r286889: 5692 expose the number of hole blocks in a file

FreeBSD porting notes:

• only kernel-side changes are merged
• the new ioctl is not actually implemented yet
• thus, the goal is to synchronize DMU code

illumos/illumos-gate@2bcf0248e992f292c7b814458bcdce2f004925d6

https://www.illumos.org/issues/5692
we would like to expose the number of hole (sparse) blocks in a file.
this can be useful to for example if you want to fill in the holes with
some data; knowing the number of holes in advances allows you to report
progress on hole filling. We could use SEEK_HOLE to do that but it would
be O(n) where n is the number of holes present in the file.

Author: Max Grossman <max.grossman@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Boris Protopopov <bprotopopov@hotmail.com>
Approved by: Richard Lowe <richlowe@richlowe.net>