sbitmap: introduce __sbitmap_for_each_set()

For blk-mq, we need to be able to iterate software queues starting
from any queue in a round robin fashion, so introduce this helper.

Reviewed-by: Omar Sandoval <osandov@fb.com>
Cc: Omar Sandoval <osandov@fb.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>