datapath: Add multicast tunnel support.

datapath: Add multicast tunnel support.

Something like this, on two separate vswitches, works to try it out:
    route add -net 224.0.0.0 netmask 240.0.0.0 dev eth0
    ovs-vsctl \
        -- add-port br0 gre0 \
        -- set interface gre0 type=gre options:remote_ip=224.0.0.1

Runtime tested on Linux 3.0, build tested on Linux 2.6.18, both i386.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>