[XFS] kill struct bhv_vfs

Now that struct bhv_vfs doesn't have any members left we can kill it and
go directly from the super_block to the xfs_mount everywhere.

SGI-PV: 969608
SGI-Modid: xfs-linux-melb:xfs-kern:29509a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>