vfs: shrink struct vnode to 448 bytes on LP64

vfs: shrink struct vnode to 448 bytes on LP64

... by moving v_hash into a 4 byte hole.

Combined with several previous size reductions this makes the size small
enough to fit 9 vnodes per page as opposed to 8.

Add a compilation time assert so that this is not unknowingly worsened.

Note the structure still remains bigger than it should be.