Import George Wilson's change for Illumos #4730:

	4730 metaslab group taskq should be destroyed in metaslab_group_destroy()
	Reviewed by: Alex Reece <alex.reece@delphix.com>
	Reviewed by: Matthew Ahrens <mahrens@delphix.com>
	Reviewed by: Sebastien Roy <sebastien.roy@delphix.com>

	Original author: George Wilson

MFC after:	3 days

(cherry picked from commit f3f864daf5d2d0f7463f79ca785b0613ada090e9)