more partition handling cleanups v2

[Christoph Hellwig <hch@xxxxxx>]

Hi Jens,

this series cleans up more lose ends in the partitioning code.  It also
removes a rather annoying use of ioctl_by_bdev in the s390 dasd driver.

Changes since v1:
 - fix a brown paperbag typo in blkpg_do_ioctl
 - fix a commit message typo