xfrm: Release dst if this dst is improper for vti tunnel

After searching rt by the vti tunnel dst/src parameter,
if this rt has neither attached to any transformation
nor the transformation is not tunnel oriented, this rt
should be released back to ip layer.

otherwise causing dst memory leakage.

Signed-off-by: Fan Du <fan.du@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>