3642 dsl_scan_active() should not issue I/O to determine if async destroying is active
3643 txg_delay should not hold the tc_lock
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Approved by: Gordon Ross <gwr@nexenta.com>
	[illumos-gate merge]

commit 4a92375985c37d61406d66cd2b10ee642eb1f5e7
    3642 dsl_scan_active() should not issue I/O to determine if async destroying is active
    3643 txg_delay should not hold the tc_lock