dnode: add dn_dirtycnt, count of number of txgs this dnode is dirty on

Bumped when we take the dirty hold in dnode_setdirty(), dropped when the
dnode is finally cleaned up after sync in dnode_rele_task() or
userquota_updates_task().

This gives us a way to check if the dnode is dirty on any txg without
having to rely on outside information (eg presence on a dirty list),
which has been a rich source of bugs in the past.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Suggested-by: Robert Evans <evansr@google.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Robert Evans <evansr@google.com>
Reviewed-by: Adam Moss <c@yotes.com>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16297
Closes #17652
Closes #17658

include/sys/dmu_impl.h

@@ -169,6 +169,7 @@ extern "C" {
  * 	dn_free_txg
  * 	dn_assigned_txg
  * 	dn_dirty_txg
+ * 	dn_dirtycnt
  * 	dd_assigned_tx
  * 	dn_notxholds
  *	dn_nodnholds

include/sys/dnode.h

@@ -341,6 +341,7 @@ struct dnode {
 	uint64_t dn_free_txg;
 	uint64_t dn_assigned_txg;
 	uint64_t dn_dirty_txg;			/* txg dnode was last dirtied */
+	uint8_t dn_dirtycnt;
 	kcondvar_t dn_notxholds;
 	kcondvar_t dn_nodnholds;
 	enum dnode_dirtycontext dn_dirtyctx;