blk-mq: Some tag allocation code refactoring

The tag allocation code to alloc the sbitmap pairs is common for regular
bitmaps tags and shared sbitmap, so refactor into a common function.

Also remove superfluous "flags" argument from blk_mq_init_shared_sbitmap().

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/1620907258-30910-2-git-send-email-john.garry@huawei.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>