vn_lock_pair(): Support passing LK_NODDLKTREAT

Since this function ultimately calls vn_lock() or VOP_LOCK1(), allows it to
receive and pass this flag which is used in the lookup code and doesn't
interfere with the function's operation.

Reviewed by: kib, markj
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D40954