If InnoDB crash recovery was needed, the InnoDB function srv_start()
would invoke extra validation, reading something from every InnoDB
data file. This should be unnecessary now that MDEV-14717 made
RENAME operations crash-safe inside InnoDB (which can be
disabled in MariaDB 10.2 by setting innodb_safe_truncate=OFF).

dict_check_sys_tables(): Skip tables that would be dropped by
row_mysql_drop_garbage_tables(). Perform extra validation only
if innodb_safe_truncate=OFF, innodb_force_recovery=0 and
crash recovery was needed.

dict_load_table_one(): Validate the root page of the table.
In this way, we can deny access to corrupted or mismatching tables
not only after crash recovery, but also after a clean shutdown.