lightnvm: introduce pr_fmt for the prefix nvm

all the pr_() family can have this prefix by pr_fmt.

Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
Reviewed-by: Javier González <javier@javigon.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>