unionfs: fix assertion order in unionfs_lock()

VOP_LOCK() may be handed a vnode that is concurrently reclaimed.
unionfs_lock() accounts for this by checking for empty vnode private
data under the interlock. But it incorrectly asserts that the vnode
is using the unionfs dispatch table before making this check.
Reverse the order, and also update KASSERT_UNIONFS_VNODE() to provide
more useful information.

Reported by: pho
Reviewed by: kib, markj, pho
Differential Revision: https://reviews.freebsd.org/D34109