FRM_MAX_SIZE too low for some use cases

As part of MDEV-6224, FRM_MAX_SIZE was increased to 512 KB. If a CREATE TABLE statement exceeds that, then the user sees an error message like the following:

Error Code: 1967. The definition for table `t` is too big

This limit is a bit low for some use cases. Especially for some Spider tables that have a lot of partitions, and which have long COMMENT strings that say on which server and which table each partition is located.