MFV r353558: 10572 10579 Fix race in dnode_check_slots_free()

MFV r353558: 10572 10579 Fix race in dnode_check_slots_free()

illumos/illumos-gate@aa02ea01948372a32cbf08bfc31c72c32e3fc81e
https://github.com/illumos/illumos-gate/commit/aa02ea01948372a32cbf08bfc31c72c32e3fc81e

10572 Fix race in dnode_check_slots_free()
https://www.illumos.org/issues/10572
  The Fix from ZoL:
  Currently, dnode_check_slots_free() works by checking dn->dn_type
  in the dnode to determine if the dnode is reclaimable. However,
  there is a small window of time between dnode_free_sync() in the
  first call to dsl_dataset_sync() and when the useraccounting code
  is run when the type is set DMU_OT_NONE, but the dnode is not yet
  evictable, leading to crashes. This patch adds the ability for
  dnodes to track which txg they were last dirtied in and adds a
  check for this before performing the reclaim.

  This patch also corrects several instances when dn_dirty_link was
  treated as a list_node_t when it is technically a multilist_node_t.

10579 Don't allow dnode allocation if dn_holds != 0
https://www.illumos.org/issues/10579
  The fix from ZoL:
  This patch simply fixes a small bug where dnode_hold_impl() could
  attempt to allocate a dnode that was in the process of being freed,
  but which still had active references. This patch simply adds the
  required check.

Author: Tom Caputi <tcaputi@datto.com>
Reported by:	delphij
MFC after:	2 weeks
X-MFC with:	r353176