nvmet: change ppl to lpp

In nvmet_bdev_set_limits() the number of logical blocks per
physical block is calculated, but the opposite is mentioned in
the associated comment and reflected in the variable name. Correct
the comment and adjust the variable name to reflect the calculation
done.

Signed-off-by: John Pittman <jpittman@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>