Done messages are missing the error code

[ghananigans]

Currently, done messages are treated as having a zero-sized payload but they are expected to have at least a 4 byte payload. This causes issues when serializing a done message to be consumed by an application expecting conformant done messages.

Per netlink message types

NLMSG_ERROR and NLMSG_DONE are of practical importance. They carry return codes for operations. 

...

NLMSG_DONE is simpler, the request is never echoed but the extended ACK attributes may be present:

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| struct nlmsghdr - response header          |
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|    int error                               |
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| ** optionally extended ACK                 |
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[ghananigans]

Created Pull request #12