Add VOP_VPUT_PAIR() with trivial default implementation.

The VOP is intended to be used in situations where VFS has two
referenced locked vnodes, typically a directory vnode dvp and a vnode
vp that is linked from the directory, and at least dvp is vput(9)ed.
The child vnode can be also vput-ed, but optionally left referenced and
locked.

There, at least UFS may need to do some actions with dvp which cannot be
done while vp is also locked, so its lock might be dropped temporary.
For instance, in some cases UFS needs to sync dvp to avoid filesystem
state that is currently not handled by either kernel nor fsck. Having
such VOP provides the neccessary context for filesystem which can do
correct locking and handle potential reclamation of vp after relock.

Trivial implementation does vput(dvp) and optionally vput(vp).

Reviewed by: chs, mckusick
Tested by: pho
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation