lightnvm: avoid memory leak when lun_map kcalloc fails

A memory leak occurs if the lower page table is initialized and the
following dev->lun_map fails on allocation.

Rearrange the initialization of lower page table to allow dev->lun_map
to fail gracefully without memory leak.

Reviewed by: Johannes Thumshirn <jthumshirn@suse.de>
Move kfree of dev->lun_map to nvm_free()
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>