MDEV-28449 Assertion `index->lock.have_x() == has_index_lock' failed in row_log_apply_op

- Alter takes index lock on commit phase twice while applying the
online log for the index build. This leads to assert failure and caused
by MDEV-15250 (4b80c11f).

storage/innobase/handler/handler0alter.cc

@@ -11050,10 +11050,14 @@ ha_innobase::commit_inplace_alter_table(
 					DBUG_RETURN(true);
 				}
 
+				index->lock.x_unlock();
+
 				error = row_log_apply(
 					m_prebuilt->trx, index, altered_table,
 					ctx->m_stage);
 
+				index->lock.x_lock(SRW_LOCK_CALL);
+
 				if (error != DB_SUCCESS) {
 					goto err_index;
 				}