lib/packet/packet_utils: Add a function to calculate fletcher checksum

I'm resending the patches which appease pep8

fletcher checksum function for OSPF LSA checksum.
refer to RFC1008.

Signed-off-by: Wataru ISHIDA <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
packet lib: add ospf

Signed-off-by: Wataru ISHIDA <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
unit test for lib.packet.ospf

Signed-off-by: Wataru ISHIDA <ishida.wataru@lab.ntt.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>