ipv4/igmp: Don't drop IGMP pkt with zeros src addr

Don't drop IGMP packets with a source address of all zeros which are
IGMP proxy reports. This is documented in Section 2.1.1 IGMP
Forwarding Rules of RFC 4541 IGMP and MLD Snooping Switches
Considerations.

Signed-off-by: Edward Chron <echron@arista.com>
Signed-off-by: David S. Miller <davem@davemloft.net>