Add some assertions for the code handling vm_object for tmpfs vnode.

Add some assertions for the code handling vm_object for tmpfs vnode.
In particular, vnode must be exclusively locked when the tmpfs vnode
and object are divorced.  When the vnode is opened, the object must be
still alive, since only live vnode can be opened, and the tmpfs node
owns a reference on the object.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks