This flag is only for the netlink protocol (multi-part messages), no reason
to reject messages without it.

Note that this flag was removed by the following kernel patches (v3.14)
65886f439ab0 ipmr: fix mfc notification flags
f518338b1603 ip6mr: fix mfc notification flags
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>