block: increase BLKCG_MAX_POLS

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.

Fixes: 08a9ad8bf6 ("block/mq-deadline: Add cgroup support")
Link: https://lore.kernel.org/lkml/8988303.mDXGIdCtx8@natalenko.name/
Signed-off-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Link: https://lore.kernel.org/r/20210717123328.945810-1-oleksandr@natalenko.name
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -57,7 +57,7 @@ struct blk_keyslot_manager;
  * Maximum number of blkcg policies allowed to be registered concurrently.
  * Defined here to simplify include dependency.
  */
-#define BLKCG_MAX_POLS		5
+#define BLKCG_MAX_POLS		6
 
 typedef void (rq_end_io_fn)(struct request *, blk_status_t);