null_blk: add configfs variables for 2 options

Allow setting via configfs these two options:

no_sched
shared_tag_bitmap

Previously these could only be activated as module parameters.

Still missing are:

shared_tags
timeout
requeue
init_hctx

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220708174943.87787-3-vincent.fu@samsung.com


[axboe: fold in nullb == NULL fix]
Signed-off-by: Jens Axboe <axboe@kernel.dk>