lightnvm: pblk: refactor rqd alloc/free

Refactor the rqd allocation and free functions so that all I/O types can
use these helper functions.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>