netlink: Split nl_req() to allow processing multiple response datagrams

Currently nl_req() sends the request, and receives a single response
datagram which we then process.  However, a single request can result in
multiple response datagrams.  That happens nearly all the time for DUMP
requests, where the 'DONE' message usually comes in a second datagram after
the NEW{LINK|ADDR|ROUTE} messages.  It can also happen if there are just
too many objects to dump in a single datagram.

Allow our netlink code to process multiple response datagrams by splitting
nl_req() into three different helpers: nl_send() just sends a request,
without getting a response.  nl_status() checks a single message to see if
it indicates the end of the reponses for our request.  nl_next() moves onto
the next response message, whether it's in a datagram we already received
or we need to recv() a new one.  We also add a 'for'-style macro to use
these to step through every response message to a request across multiple
datagrams.

While we're at it, be more thourough with checking that our sequence
numbers are in sync.

Link: https://bugs.passt.top/show_bug.cgi?id=67
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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