vxlan: interpret IP headers for ECN correctly

When looking for outer IP header, use the actual socket address family, not
the address family of the default destination which is not set for metadata
based interfaces (and doesn't have to match the address family of the
received packet even if it was set).

Fix also the misleading comment.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>