MDEV-33214 Table is getting rebuild with ALTER TABLE ADD COLUMN

Problem:
======
- InnoDB fail to do instant operation while adding the variable
length column. Problem is that InnoDB wrongly assumes that
variable character length can never part of externally stored
page.

Solution:
========
instant_alter_column_possible(): Variable length
character field can be stored as externally stored page.

mysql-test/suite/innodb/t/instant_alter_extend.test

@@ -256,3 +256,16 @@ select * from t1;
 check table t1;
 
 drop database best;
+
+--echo #
+--echo # MDEV-33214 Table is getting rebuild with
+--echo #         ALTER TABLE ADD COLUMN
+--echo #
+use test;
+CREATE TABLE t1(f1 INT, f2 VARCHAR(10)) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_general_ci;
+INSERT INTO t1 VALUES(1,'abc'),(2,'def');
+ALTER TABLE t1 ADD (f3 VARCHAR(5000), f4 VARCHAR(20)), ALGORITHM=instant;
+ALTER TABLE t1 ADD f5 TEXT, ALGORITHM=INSTANT;
+DROP TABLE t1;
+
+--echo # End of 10.4 tests

storage/innobase/handler/handler0alter.cc

@@ -1581,11 +1581,9 @@ instant_alter_column_possible(
 			ut_ad(!is_null || nullable);
 			n_nullable += nullable;
 			n_add++;
-			uint l;
+			uint l = (*af)->pack_length();
 			switch ((*af)->type()) {
 			case MYSQL_TYPE_VARCHAR:
-				l = reinterpret_cast<const Field_varstring*>
-					(*af)->get_length();
 			variable_length:
 				if (l >= min_local_len) {
 					max_size += blob_prefix
@@ -1599,7 +1597,6 @@ instant_alter_column_possible(
 					if (!is_null) {
 						min_size += l;
 					}
-					l = (*af)->pack_length();
 					max_size += l;
 					lenlen += l > 255 ? 2 : 1;
 				}
@@ -1613,7 +1610,6 @@ instant_alter_column_possible(
 					((*af))->get_length();
 				goto variable_length;
 			default:
-				l = (*af)->pack_length();
 				if (l > 255 && ib_table.not_redundant()) {
 					goto variable_length;
 				}