[XFS] kill the v_number member in struct bhv_vnode

It's entirely unused except for ignored arguments in the mrlock
initialization, so remove it.

SGI-PV: 969608
SGI-Modid: xfs-linux-melb:xfs-kern:29499a

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