lightnvm: refactor target type lookup

Refactor target type lookup to use/not use locks explicitly instead of
using a hidden parameter to make the function locking.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>