Acquire a hold reference on the vnode when a knote is instantiated.
Otherwise, knote keeps a pointer to a vnode which could become invalid
any time.

Reported by:	many
Tested by:	Patrick Lamaiziere <patfbsd@davenulle.org>
Discussed with:	jmg
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (marius)