add a ->free_disk block_device_operation v2

Hi Jens,

this series adds a ->free_disk method to struct block_device_operation so that
drivers can defer freeing their disk private data until the gendisk goes away
and don't need to play games with the validity of ->private_data.

This also converts three simple drivers over as example, but eventually I
imagine that all drivers with private data will use it.

Changes since v1:
 - fix a tag_set use after free in virtio_blk

Diffstat:
 block/genhd.c                       |    4 +
 drivers/block/virtio_blk.c          |   74 ++++++++++--------------------------
 drivers/memstick/core/ms_block.c    |   64 +++++++------------------------
 drivers/memstick/core/ms_block.h    |    1 
 drivers/memstick/core/mspro_block.c |   57 ++++-----------------------
 include/linux/blkdev.h              |    1 
 6 files changed, 51 insertions(+), 150 deletions(-)