vfs_register_upper_from_vp
xmp->nullm_vfs = vfs_register_upper_from_vp(lowerrootvp, mp,
ump->um_lowermp = vfs_register_upper_from_vp(ump->um_lowervp, mp,
ump->um_uppermp = vfs_register_upper_from_vp(ump->um_uppervp, mp,
struct mount *vfs_register_upper_from_vp(struct vnode *,