lightnvm: refactor dev->online_target to global nvm_targets

A target name must be unique. However, a per-device registration of
targets is maintained on a dev->online_targets list, with a per-device
search for targets upon registration.

This results in a name collision when two targets, with the same name,
are created on two different targets, where the per-device list is not
shared.

Signed-off-by: Simon A. F. Lund <slund@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>