Problem:
========
 - Drop table assert if innodb_force_recovery is set to 5 or 6.
For innodb_force_recovery 5 and 6, InnoDB doesn't scan the undo log
and it makes the redo rollback segment as NULL. There is no way for
transaction to write any undo log.

 - If innodb_force_recovery is set to 6 then InnoDB does not do the
redo log roll-forward in connection with recovery. In this case,
log_sys will be initalized only and it will not have latest
checkpoint information. Checkpoint is done during shutdown even
innodb_force_recovery is set to 6. So it leads to incorrect
information update in checkpoint header.

Solution:
========
1) Allow drop table only if innodb_force_recovery < 5.
2) Make innodb as read-only if innodb_force_recovery is set to 6.
3) During shutdown, remove the checkpoint if innodb_force_recovery
is set to 6.
Reviewed-by: Jimmy Yang <jimmy.yang@oracle.com>
RB: 15075