Please take a look at these three patches for 4.4-rc2.
The first patch uses a slab allocator for ppa list to only allocate the
necessary memory. As the linear addressing mode was removed, the second
patch changes the driver to use the ppa format adressing mode and at
last, the third patch fixes two bugs. One when freeing gendisk on unload
and another to pass a null pointer to lightnvm unregister function.
Feel free to fold them into the original null_blk patch.