lightnvm: pblk: check for chunk size before allocating it

Do the check for the chunk state after making sure that the chunk type
is supported.

Fixes: 32ef9412 ("lightnvm: pblk: implement get log report chunk")
Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>