null_blk: cleanup null_init_tag_set

null_blk: cleanup null_init_tag_set

The passed 'nullb' can be NULL, so cause null ptr reference.

Fix the issue, meantime cleanup null_init_tag_set for avoiding to add
similar issue in future.

Meantime set BLK_MQ_F_NO_SCHED if g_no_sched is true in case of NULL
device, same with BLK_MQ_F_TAG_HCTX_SHARED.

Cc: Vincent Fu <vincent.fu@samsung.com>
Fixes: 37ae152c7a0d ("null_blk: add configfs variables for 2 options")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220715142847.188275-1-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>