mac-learning: Increase default mac table size to 8K from 2K

mac-learning: Increase default mac table size to 8K from 2K

In field deployments of OVS (mostly in combination with OpenStack) we
see that the 2K default MAC forwarding table is too small.

On average this tables is around 5k entries, hence this patch to
increase the default value to the next power of 2, i.e. 8K.

This increase in size does not automatically increase the memory
footprint, as the memory for the MAC entries, are allocated only when
needed.

Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>