vnode: Assert that VOP_LOCK succeeds in freevnode()

Reviewed by:	kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D52245

Author: markjdb

sys/kern/vfs_subr.c

@@ -2186,6 +2186,8 @@ freevnode(struct vnode *vp)
 {
 	struct bufobj *bo;
 
+	ASSERT_VOP_UNLOCKED(vp, __func__);
+
 	/*
 	 * The vnode has been marked for destruction, so free it.
 	 *
@@ -2222,12 +2224,16 @@ freevnode(struct vnode *vp)
 	mac_vnode_destroy(vp);
 #endif
 	if (vp->v_pollinfo != NULL) {
+		int error __diagused;
+
 		/*
 		 * Use LK_NOWAIT to shut up witness about the lock. We may get
 		 * here while having another vnode locked when trying to
 		 * satisfy a lookup and needing to recycle.
 		 */
-		VOP_LOCK(vp, LK_EXCLUSIVE | LK_NOWAIT);
+		error = VOP_LOCK(vp, LK_EXCLUSIVE | LK_NOWAIT);
+		VNASSERT(error == 0, vp,
+		    ("freevnode: cannot lock vp %p for pollinfo destroy", vp));
 		destroy_vpollinfo(vp->v_pollinfo);
 		VOP_UNLOCK(vp);
 		vp->v_pollinfo = NULL;