block: Catch possible entries missing from alloc_policy_name[]

Make BLK_TAG_ALLOC_x an enum and add a "max" entry.

Add a BUILD_BUG_ON() call to ensure that we are not missing entries in
hctx_flag_name[].

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20240719112912.3830443-12-john.g.garry@oracle.com


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