lightnvm: check mm before use

The core can may issue I/Os before a media manager is registered with
the lightnvm subsystem. Make sure that we don't call the media manager
->end_io prematurely with a null pointer.

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