event/dsw: avoid migration waves in large systems

event/dsw: avoid migration waves in large systems

DSW limits the rate of migrations on a per-port basis. Hence, as the
number of cores grows, so does the total migration capacity.

In high core-count systems, this allows for a situation where flows
are migrated to a lightly loaded port which recently already received
a number of new flows (from other ports). The processing load
generated by these new flows may not yet be reflected in the lightly
loaded port's load estimate. The result is that the previously lightly
loaded port is now overloaded.

This patch adds a rough estimate of the size of the inbound migrations
to a particular port, which can be factored into the migration logic,
avoiding the above problem.

Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>