Allow ZVOL bookmarks to be listed recursively

Allow ZVOL bookmarks to be listed recursively

Many thanks to cryx-freebsd@h3q.com for reporting the problem and
submitting a fix.  I have chosen to take an equivalent but textually
different patch from ZoL just to avoid increasing divergence between
OpenZFS flavours.

ZoL commit:	zfsonlinux/zfse33da554c5daf0103b093f44ab5b90ad6c064c3f
Author:		loli10K <ezomori.nozomu@gmail.com>
Date:		Wed Sep 7 19:34:20 2016 +0200
PR:		197821
Submitted by:	cryx-freebsd@h3q.com (alternative version)
Reported by:	cryx-freebsd@h3q.com
Obtained from:	ZoL
MFC after:	1 week