After mq-deadline learned to deal with cgroups, the BLKCG_MAX_POLS value
became too small for all the elevators to be registered properly. The
following issue is seen:
```
calling bfq_init+0x0/0x8b @ 1
blkcg_policy_register: BLKCG_MAX_POLS too small
initcall bfq_init+0x0/0x8b returned -28 after 507 usecs
```
which renders BFQ non-functional.
Increase BLKCG_MAX_POLS to allow enough space for everyone.