Check for mismatched vref()/vdrop()

Assert that the hold count has not fallen below the use count, a situation
that would only happen when a vref() (or similar) is erroneously paired
with a vdrop().  This situation has not been observed in the wild, but
could be helpful for someone implementing a new filesystem.

Reviewed by:	kib
Approved by:	hrs (mentor)

sys/kern/vfs_subr.c

@@ -2343,6 +2343,8 @@ vdropl(struct vnode *vp)
 	if (vp->v_holdcnt <= 0)
 		panic("vdrop: holdcnt %d", vp->v_holdcnt);
 	vp->v_holdcnt--;
+	VNASSERT(vp->v_holdcnt >= vp->v_usecount, vp,
+	    ("hold count less than use count"));
 	if (vp->v_holdcnt > 0) {
 		VI_UNLOCK(vp);
 		return;