block: move sysfs methods shared by disks and partitions to genhd.c

Move the sysfs _show methods that are used both on the full disk and
partition nodes to genhd.c instead of hiding them in the partitioning
code.  Also move the declaration for these methods to block/blk.h so
that we don't expose them to drivers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>