net: Do not wrap sysctl igmp_max_memberships in IP_MULTICAST

controlling igmp_max_membership is useful even when IP_MULTICAST
is off.
Quagga(an OSPF deamon) uses multicast addresses for all interfaces
using a single socket and hits igmp_max_membership limit when
there are 20 interfaces or more.
Always export sysctl igmp_max_memberships in proc, just like
igmp_max_msf

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: David S. Miller <davem@davemloft.net>