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

include/sys/dnode.h

@@ -260,6 +260,7 @@ struct dnode {
 	uint64_t dn_allocated_txg;
 	uint64_t dn_free_txg;
 	uint64_t dn_assigned_txg;
+	uint64_t dn_dirty_txg;			/* txg dnode was last dirtied */
 	kcondvar_t dn_notxholds;
 	enum dnode_dirtycontext dn_dirtyctx;
 	uint8_t *dn_dirtyctx_firstset;		/* dbg: contents meaningless */
@@ -362,6 +363,9 @@ void dnode_evict_dbufs(dnode_t *dn);
 void dnode_evict_bonus(dnode_t *dn);
 void dnode_free_interior_slots(dnode_t *dn);
 
+#define	DNODE_IS_DIRTY(_dn)						\
+	((_dn)->dn_dirty_txg >= spa_syncing_txg((_dn)->dn_objset->os_spa))
+
 #define	DNODE_IS_CACHEABLE(_dn)						\
 	((_dn)->dn_objset->os_primary_cache == ZFS_CACHE_ALL ||		\
 	(DMU_OT_IS_METADATA((_dn)->dn_type) &&				\