You shouldn't need the overhead of GRE just to add multicast
capability on a point-to-point interface (for instance, you might
want to run mDNS over IPsec transport connections, and Avahi
requires IFF_MULTICAST be set on interfaces, even point-to-point
ones).
Borrowed heavily from:
b3c9321b9e gre: Support multicast configurable gre interfaces
Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
2538759688