rstp: shift learned MAC addresses to new Root port.

rstp: shift learned MAC addresses to new Root port.

All MAC addresses previously learned on a Root Port can be moved to an
Alternate Port that becomes the new Root Port; i.e., Dynamic Filtering
Entries for those addresses may be modified to show the new Root Port as
their source, reducing the need to flood frames when recovering from
some component failures.

Signed-off-by: Daniele Venturino <daniele.venturino@m3s.it>
Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>