blk-mq: use HCTX_TYPE_DEFAULT but not 0 to index blk_mq_tag_set->map

Replace set->map[0] with set->map[HCTX_TYPE_DEFAULT] to avoid hardcoding.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>