unionfs: implement VOP_VPUT_PAIR

unionfs must pass VOP_VPUT_PAIR directly to the underlying FS so that
it can have a chance to manage any special locking considerations that
may be necessary. The unionfs implementation is based heavily on the
corresponding nullfs implementation.

Also note some outstanding issues with the unionfs locking scheme, as
a first step in fixing those issues in a future change.

Discussed with: kib
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D33008