[XFS] decontaminate vfs operations from behavior details

All vfs ops now take struct xfs_mount pointers and the behaviour related
glue is split out into methods of its own.

SGI-PV: 969608
SGI-Modid: xfs-linux-melb:xfs-kern:29504a

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