lightnvm: fix off-by-one error on target initialization

If one specifies the end lun id to be the absolute number of luns,
without taking zero indexing into account, the lightnvm core will pass
the off-by-one end lun id to target creation, which then panics during
nvm_ioctl_dev_create.

Signed-off-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@fb.com>