lightnvm: add check after mempool allocation

The mempool allocation might fail. Make sure to return error when it
does, instead of causing a kernel panic.

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