[PATCH] Add bdev private field

[PATCH] Add bdev private field

From: Christoph Hellwig <hch@lst.de>

Currently xfs has a per-bdev (and XFS filesystem uses up to three underlying
block devices) object that Al complained about loudly that it should be gone.

But for that to happen without rewriting half of XFS (and changing layering
in a way that we don't really want) we need an additional fs-private variable
in struct block_device.