unionfs_lookup(): fix wild accesses to vnode private data

There are a few spots in which unionfs_lookup() accesses unionfs vnode
private data without holding the corresponding vnode lock or interlock.

Reviewed by: kib, olce
Differential Revision: https://reviews.freebsd.org/D44601

(cherry picked from commit b18029bc59d2ed6b0eeeb233189cf713b34b467c)