MFC r309425,r312645,r312598,r312621:

vfs: provide fake locking primitives for the crossmp vnode

Since the vnode is only expected to be shared locked, we can save a
little overhead by only pretending we are locking in the first place.

Provide fallback VOP methods for crossmp vnode.

In particular, crossmp vnode might leak into rename code.

vfs: hide the getvnode NULL mp message behind DIAGNOSTIC

Since crossmp vnode changes the message was being printed on each boot.

Improve debugging printf.