The OpenFlow specification says that only role changes to slave or master
check the generation ID, so this is a bug fix.

OpenFlow 1.2 section A.3.9 phrases the requirement this way:

    Additionally, if the role value in the message is OFPCR_ROLE_MASTER
    or OFPCR_ROLE_SLAVE, the switch must validate generation_id to check
    for stale messages.

Found by OFTest.

Signed-off-by: Ben Pfaff <blp@nicira.com>