dbuf: Handle arcbuf assignment after block cloning

In some cases dbuf_assign_arcbuf() may be called on a block that
was recently cloned.  If it happened in current TXG we must undo
the block cloning first, since the only one dirty record per TXG
can't and shouldn't mean both cloning and overwrite same time.

Reviewed-by: Kay Pedersen <mail@mkwg.de>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#15653

module/zfs/dbuf.c

@@ -2945,7 +2945,8 @@ dbuf_assign_arcbuf(dmu_buf_impl_t *db, arc_buf_t *buf, dmu_tx_t *tx)
 	while (db->db_state == DB_READ || db->db_state == DB_FILL)
 		cv_wait(&db->db_changed, &db->db_mtx);
 
-	ASSERT(db->db_state == DB_CACHED || db->db_state == DB_UNCACHED);
+	ASSERT(db->db_state == DB_CACHED || db->db_state == DB_UNCACHED ||
+	    db->db_state == DB_NOFILL);
 
 	if (db->db_state == DB_CACHED &&
 	    zfs_refcount_count(&db->db_holds) - 1 > db->db_dirtycnt) {
@@ -2982,6 +2983,15 @@ dbuf_assign_arcbuf(dmu_buf_impl_t *db, arc_buf_t *buf, dmu_tx_t *tx)
 			arc_buf_destroy(db->db_buf, db);
 		}
 		db->db_buf = NULL;
+	} else if (db->db_state == DB_NOFILL) {
+		/*
+		 * We will be completely replacing the cloned block.  In case
+		 * it was cloned in this transaction group, let's undirty the
+		 * pending clone and mark the block as uncached. This will be
+		 * as if the clone was never done.
+		 */
+		VERIFY(!dbuf_undirty(db, tx));
+		db->db_state = DB_UNCACHED;
 	}
 	ASSERT(db->db_buf == NULL);
 	dbuf_set_data(db, buf);