MDEV-16288 ALTER TABLE…ALGORITHM=DEFAULT does not override alter_algorithm

- ALTER_ALGORITHM should be substituted when there is no mention of
algorithm in alter statement.
- Introduced algorithm(thd) in Alter_info. It returns the
user requested algorithm. If user doesn't specify algorithm explicitly then
it returns alter_algorithm variable.
- changed algorithm() to get_algorithm(thd) to return algorithm name for
displaying the error.
- set_requested_algorithm(algo_value) to avoid direct assignment on
requested_algorithm variable.
- Avoid direct access of requested_algorithm to encapsulate
requested_algorithm variable

sql/sql_partition.cc

@@ -6008,7 +6008,7 @@ the generated partition syntax in a correct manner.
         */
         if (alter_info->partition_flags != ALTER_PARTITION_INFO ||
             !table->part_info ||
-            alter_info->requested_algorithm !=
+            alter_info->algorithm(thd) !=
               Alter_info::ALTER_TABLE_ALGORITHM_INPLACE ||
             !table->part_info->has_same_partitioning(part_info))
         {