[IPV4] igmp: Fixup struct ip_mc_list::multiaddr type

All users except two expect 32-bit big-endian value. One is of

	->multiaddr = ->multiaddr

variety. And last one is "%08lX".

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>