block: null_blk: Use the bitmap API to allocate bitmaps

Use bitmap_zalloc()/bitmap_free() instead of hand-writing them.

It is less verbose and it improves the semantic.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/7c4d3116ba843fc4a8ae557dd6176352a6cd0985.1656864320.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jens Axboe <axboe@kernel.dk>