From: viro@parcelfarce.linux.theplanet.co.uk <viro@parcelfarce.linux.theplanet.co.uk>

For bdevfs inodes (ones created along with struct block_device by
fs/block_dev.c) we have inode->i_bdev equal to &BDEV_I(inode)->bdev (i.e.
it's at the constant offset from inode).  New helper added for such inodes
(I_BDEV(inode)).  A bunch of places (mostly in block_dev.c) switched to use
of that helper.  A bunch of places that used

	file->f_dentry->d_inode->i_bdev->bd_inode

switched to

	file->f_mapping->host

- those expressions are equal whenever the former is valid.