sctp: Optimize computation of highest new tsn in SACK.

Right now, if the highest tsn in the SACK doesn't change, we'll
end up scanning the transmitted lists on the transports twice:
once for locating the highest _new_ tsn, and once for actually
tagging chunks as acked.  This is a waste, since we can record
the highest _new_ tsn at the same time as tagging chunks.  Long
ago this was not possible because we would try to mark chunks
as missing at the same time as tagging them acked and this approach
didn't work.  Now that the two steps are separate, we can re-use
the old approach.

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>