sbitmap: add helpers for dumping to a seq_file

This is useful debugging information that will be used in the blk-mq
debugfs directory.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>

Changed 'weight' to 'busy'.

Signed-off-by: Jens Axboe <axboe@fb.com>