block: move struct partition out of genhd.h

struct partition is the on-disk format of a MSDOS partition table entry.
Move it out of genhd.h into a new msdos_partition.h header and give it
a msdos_ prefix to avoid confusion.
Also move the magic number from block/partitions/msdos.h to the new
header so that it can be used by the SCSI drivers looking at the DOS
partition tables.

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