lightnvm: fixes to nvm_exit

Exported 'nvm_exit' to make it callable from outside modules and
corrected the deallocation call of pool->blocks from kfree to
vfree to avoid crashing on deallocation.
null_blk: call nvm_exit on unload iff LNVM device

nvm_exit, responsible for cleanup of lightnvm allocations and pre-
shutdown procedures, was never called before.
nvme: call nvm_exit on unload iff LNVM device

nvm_exit, responsible for cleanup of lightnvm allocations and pre-
shutdown procedures, was never called before.