block: move the various x86 Unix label formats out of genhd.h

All these are just used in block/partitions/msdos.c, so move them out of the
genhd.h driver included by every driver.

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