block: refactor blkpg_ioctl

Split each sub-command out into a separate helper, and move those helpers
to block/partitions/core.c instead of having a lot of partition
manipulation logic open coded in block/ioctl.c.

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