Log zvol truncate/discard operations

As the comments in zvol_discard() suggested, the discard operation
could be logged to the zil. This is a port of the relevant code from
Nexenta as it was added in "701 UNMAP support for COMSTAR" and has been
attributed to the author of that commit.

References:

https://github.com/Nexenta/illumos-nexenta/commit/b77b923
https://github.com/zfsonlinux/zfs/blob/089fa91b/module/zfs/zvol.c#L637

Ported-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>