datapath: Extend compatibility code for genl_register_mc_group().

datapath: Extend compatibility code for genl_register_mc_group().

The existing compatibility code for genl_register_mc_group() always returns
the same value because the only caller (in brcompat_mod) only needs a
single multicast group.  However, when the datapath is converted over to
using Netlink in an upcoming commit, openvswitch_mod will start needing a
number of multicast groups, so this commit adds this ability.

The multicast group ranges differ for brcompat_mod and openvswitch_mod so
that they don't interfere with one another.  (This would waste time in
ovs-brcompatd and ovs-vswitchd, although it would not be fatal.)

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