lightnvm: add a bitmap of luns

Add a bitmap of luns to indicate the status
of luns: inuse/available. When create targets
do the necessary check to avoid allocating luns
that are already allocated.

Signed-off-by: Wenwei Tao <ww.tao0320@gmail.com>
Freed dev->lun_map if nvm_core_init later failed in the init process.
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>