- When foreign_key_check is disabled, allowing to modify the
column which is part of foreign key constraint can lead to
refusal of TRUNCATE TABLE, OPTIMIZE TABLE later. So it make
sense to block the column modify operation when foreign key
is involved irrespective of foreign_key_check variable.

Correct way to modify the charset of the column when fk is involved:

SET foreign_key_checks=OFF;
ALTER TABLE child DROP FOREIGN KEY fk, MODIFY m VARCHAR(200) CHARSET utf8mb4;
ALTER TABLE parent MODIFY m VARCHAR(200) CHARSET utf8mb4;
ALTER TABLE child ADD CONSTRAINT FOREIGN KEY (m) REFERENCES PARENT(m);
SET foreign_key_checks=ON;

fk_check_column_changes(): Remove the FOREIGN_KEY_CHECKS while
checking the column change for foreign key constraint. This
is the partial revert of commit 5f1f2fc0
and it changes the behaviour of copy alter algorithm

ha_innobase::prepare_inplace_alter_table(): Find the modified
column and check whether it is part of existing and newly
added foreign key constraint.