block: simplify tag allocation policy selection

Use a plain BLK_MQ_F_* flag to select the round robin tag selection
instead of overlaying an enum with just two possible values into the
flags space.

Doing so allows adding a BLK_MQ_F_MAX sentinel for simplified overflow
checking in the messy debugfs helpers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20250106083531.799976-5-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>