Fix race in dnode_check_slots_free()

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.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #7147 
Closes #7388

include/sys/dmu_impl.h

@@ -161,6 +161,7 @@ extern "C" {
  * 	dn_allocated_txg
  * 	dn_free_txg
  * 	dn_assigned_txg
+ * 	dn_dirty_txg
  * 	dd_assigned_tx
  * 	dn_notxholds
  * 	dn_dirtyctx