lightnvm: remove nested lock conflict with mm

If a media manager tries to initialize it targets upon media manager
initialization, the media manager will need to know which target types
are available in LightNVM. The lists of which managers and target types
are available shares the same lock.

Therefore, on initialization, the nvm_lock is taken by LightNVM core,
which later leads to a deadlock when target types are enumerated by the
media manager.

Add an exclusive lock for target types to resolve this conflict.

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