tap: Consider IPv6 flow label when building packet sequences

To allow more batching, we group together related packets into "seqs" in
the tap layer, before passing them to the L4 protocol layers.  Currently
we consider the IP protocol, both IP addresses and also the L4 ports when
grouping things into seqs.  We ignore the IPv6 flow label.

We have some future cases where we want to consider the the flow label in
the L4 code, which is awkward if we could be given a single batch with
multiple labels.  Add the flow label to tap6_l4_t and group by it as well
as the other criteria.  In future we could possibly use the flow label
_instead_ of peeking into the L4 header for the ports, but we don't do so
for now.

The guest should use the same flow label for all packets in a low, but if
it doesn't this change won't break anything, it just means we'll batch
things a bit sub-optimally.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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