MFC r251635: illumos #3747 txg commit callbacks don't work

MFC r251635: illumos #3747 txg commit callbacks don't work

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/txg.c:
	Fix commit callbacks by moving them to the task's list.
	Previously, list_move_tail() returned without doing anything because
	the task list was passed as the source rather than destination.

cddl/contrib/opensolaris/cmd/ztest/ztest.c:
	Check the commit callback threshold correctly.

Submitted by:	will
Reviewed by:	Matthew Ahrens <mahrens@delphix.com>,
		Christopher Siden <christopher.siden@delphix.com>
Sponsored by:	Spectra Logic

git-svn-id: svn://svn.freebsd.org/base/stable/8@252761 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f