unionfs: Improve vnode validation

unionfs: Improve vnode validation

Instead of validating that a vnode belongs to unionfs only when the
caller attempts to extract the upper or lower vnode pointers, do this
validation any time the caller tries to extract a unionfs_node from
the vnode private data.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D32629