Fix dirty check in dmu_offset_next()

The correct way to determine if a dnode is dirty is to check
if any of the dn->dn_dirty_link's are active.  Relying solely
on the dn->dn_dirtyctx can result in the dnode being mistakenly
reported as clean.

Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3125 
Closes #6867

module/zfs/dmu.c

@@ -2250,12 +2250,10 @@ dmu_offset_next(objset_t *os, uint64_t object, boolean_t hole, uint64_t *off)
 	/*
 	 * Check if dnode is dirty
 	 */
-	if (dn->dn_dirtyctx != DN_UNDIRTIED) {
-		for (i = 0; i < TXG_SIZE; i++) {
-			if (!list_is_empty(&dn->dn_dirty_records[i])) {
-				clean = B_FALSE;
-				break;
-			}
+	for (i = 0; i < TXG_SIZE; i++) {
+		if (list_link_active(&dn->dn_dirty_link[i])) {
+			clean = B_FALSE;
+			break;
 		}
 	}