Add free block, used block, and bad block information to the show debug
interface. This information is used to debug how targets track blocks.

Also, change debug function name to make it more generic.
Signed-off-by: NJavier Gonzalez <javier@cnexlabs.com>
Signed-off-by: NMatias Bjørling <m@bjorling.me>
Signed-off-by: NJens Axboe <axboe@fb.com>