Bugfix: incorrect calculation on IP packet header length

[wenyingd]

This change is to resolve this issue: the header option length in an IP packet header is not counted.

There are at least 20 bytes in the IP packet header. Hence, if the IHL value is less than 5, 5 is used to count IP header length, otherwise it should use the IHL value directly.

[ceclinux]

I think the title should be "Bugfix: incorrect calculation on IP packet header length". You should fix the commit message as well.

[tnqn]

Could you merge https://github.com/antrea-io/libOpenflow/pull/39 first before this one so we can use the new tag?

[ceclinux]

LGTM

[wenyingd]

Could you merge #39 first before this one so we can use the new tag?

Done.