lightnvm: pblk: put bio on bio completion

Simplify put bio by doing it on bio end_io instead of manually putting
it on the completion path.

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