block: Remove the ALLOC_CACHE_SLACK constant

Commit b99182c5 ("bio: add pcpu caching for non-polling bio_put")
removed the code that uses this constant. Hence also remove the constant
itself.

Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20230209230135.3475829-1-bvanassche@acm.org


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