vn_lock_pair(): reasonably handle vp1 == vp2 case

Lock the vnode in the most exclusive lock mode requested, once.
All callers already ensure that vp1 != vp2 or are careful enough to only
unlock once otherwise.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D42642