• Add KTR_VFS tracing to track the life of vnodes. Eventually KTR_VFS events could be added to cover other interesting details.
• Add some VNASSERTs to discover places where we access vnodes after they have been uma_zfree'd before we try to free them again.
• Add a few more VNASSERTs to vdestroy() to be certain that the vnode is really unused.

Sponsored by: Isilon Systems, Inc.