Holding a vnode doesn't prevent v_mount from disappearing (when the

Holding a vnode doesn't prevent v_mount from disappearing (when the
vnode is inactivated), possibly leading to a NULL dereference when
checking if the mount wants knotes to be activated in the VOP hooks.
So, we add a new vnode flag VV_NOKNOTE that is only set in getnewvnode(),
if necessary, and check it when activating knotes.
Since the flags are not erased when a vnode is being held, we can safely
read them.

Reviewed by:	kris@
MFC after:	3 days