lightnvm: pblk: implement get log report chunk

In preparation of pblk supporting 2.0, implement the get log report
chunk in pblk. Also, define the chunk states as given in the 2.0 spec.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>