vlrureclaim: only skip vnode with resident pages if it own the pages

Nullfs vnode which shares vm_object and pages with the lower vnode should
not be exempt from the reclaim just because lower vnode cached a lot.
Their reclamation is actually very cheap and should be preferred over
real fs vnodes, but this change is already useful.

Reported and tested by:	pho
Reviewed by:	mckusick
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D29178

sys/kern/vfs_subr.c

@@ -1169,7 +1169,7 @@ vlrureclaim(bool reclaim_nc_src, int trigger, u_long target)
 		VI_LOCK(vp);
 		if (vp->v_usecount > 0 ||
 		    (!reclaim_nc_src && !LIST_EMPTY(&vp->v_cache_src)) ||
-		    (vp->v_object != NULL &&
+		    (vp->v_object != NULL && vp->v_object->handle == vp &&
 		    vp->v_object->resident_page_count > trigger)) {
 			VOP_UNLOCK(vp);
 			vdropl(vp);