Users that wish to iterate all luns on a device. Must create a
struct ppa_addr and separate iterators for channels and luns. To set the
iterators, two loops are required, one to iterate channels, and another
to iterate luns. This leads to decrease in readability.

Introduce nvm_for_each_lun_ppa, which implements the nested loop and
sets ppa, channel, and lun variable for each loop body, eliminating
the boilerplate code.
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>