lightnvm: reference rrpc lun in rrpc block

Currently, a rrpc block only points to its nvm_lun. If a user wants to
find the associated rrpc lun, it will have to calculate the index and
look it up manually. By referencing the rrpc lun directly, this step can
be omitted, at the cost of a larger memory footprint.

This is important for upcoming patches that implement write buffering in
rrpc.

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