add_assertions

Now that v_usecount tracks valid references only add some "v_usecount == 1"
assertions.

diff -r cd6b1075f2e3 -r c8577cb16b57 sys/kern/vfs_vnode.c
--- a/sys/kern/vfs_vnode.c	Tue Dec 27 16:32:36 2016 +0100
+++ b/sys/kern/vfs_vnode.c	Tue Dec 27 16:32:36 2016 +0100
@@ -304,6 +304,10 @@
 	if (node->vi_state != from)
 		vnpanic(vp, "from is %s, expected %s at %s:%d\n",
 		    vstate_name(node->vi_state), vstate_name(from), func, line);
+	if ((from == VS_BLOCKED || to == VS_BLOCKED) && vp->v_usecount != 1)
+		vnpanic(vp, "%s to %s with usecount %d at %s:%d",
+		    vstate_name(from), vstate_name(to), vp->v_usecount,
+		    func, line);
 
 	node->vi_state = to;
 	if (from == VS_LOADING)
@@ -877,6 +881,7 @@
 	mutex_enter(vp->v_interlock);
 	VSTATE_CHANGE(vp, VS_BLOCKED, VS_ACTIVE);
 
+	KASSERT(vp->v_usecount == 1);
 	vcache_reclaim(vp);
 	vrelel(vp, 0);