lightnvm: do not assume sequential lun alloc.

When doing GC, rrpc calculates the physical LUN to which the rrpc block
belongs too. This calculation is based on the assumption that LUNs are
assigned sequentially to the LUN list. Use the reference to the LUN
instead. This saves us the calculation and allows us to align LUNs in a
different manner to, for example, take advantage of devide parallelism.

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