RFC: sort out get_gendisk abuses

[Christoph Hellwig]

Hi all,

this series tries to remove two abuses of the get_gendisk API.
The first one is fairly straigt forward and switched the blk-cgroup
configuration API to properly open the block device, but I'd love to see
it reviewed and tested by the cgroup maintainers, as I don't really know
how this code is actually used.

The other one in the hibernation code really puzzles me - it busy loops
on the gendisk lookup, just to leak a reference when it finally succeeds
and then doesn't actually do anything with the result.  My proposal for
now is to simply revert the offending change, but better proposals are
welcome.