lightnvm: fix media mgr registration

This patch fixes two issues during media manager registration.

1. The ppa pool can be used at media manager registration. Allocate the
ppa pool before that.

2. If a media manager can't be found, this should not lead to the
device being unallocated. A media manager can be registered later, that
can manage the device. Only warn if a media manager fails
initialization.

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