Illumos #3642, #3643

Illumos #3642, #3643

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>

References:
  https://www.illumos.org/issues/3642
  https://www.illumos.org/issues/3643
  illumos/illumos-gate@4a92375985c37d61406d66cd2b10ee642eb1f5e7

Ported-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1775

Porting Notes:

1. The alignment assumptions for the tx_cpu structure assume that
   a kmutex_t is 8 bytes.  This isn't true under Linux but tc_pad[]
   was adjusted anyway for consistency since this structure was
   never carefully aligned in ZoL.  If careful alignment does impact
   performance significantly this should be reworked to be portable.