block: partition: introduce hd_free_part()

So the helper can be used in both generic partition
case and part0 case.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --cc block/genhd.c
Simple merge

diff --cc block/partition-generic.c
Simple merge

diff --cc include/linux/genhd.h
Simple merge