[XFS] remove xfs_vfs.h

The only thing left are the forced shutdown flags and freeze macros which
fit into xfs_mount.h much better.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>