ipmr: fix sparse warning when testing origin or group

mfc_mcastgrp and mfc_origin are __be32, thus we need to convert INADDR_ANY.
Because INADDR_ANY is 0, this patch just fix sparse warnings.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>