igmp: hash a hash table to speedup ip_check_mc_rcu()

igmp: hash a hash table to speedup ip_check_mc_rcu()

After IP route cache removal, multicast applications using
a lot of multicast addresses hit a O(N) behavior in ip_check_mc_rcu()

Add a per in_device hash table to get faster lookup.

This hash table is created only if the number of items in mc_list is
above 4.

Reported-by: Shawn Bohrer <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Tested-by: Shawn Bohrer <[email protected]>
Reviewed-by: Cong Wang <[email protected]>
Signed-off-by: David S. Miller <[email protected]>