igmp: add __ip_mc_{join|leave}_group()

There is a need to perform igmp join/leave operations while RTNL is
held.

Make ip_mc_{join|leave}_group() wrappers around
__ip_mc_{join|leave}_group() to avoid the proliferation of work queues.

For example, vxlan_igmp_join() could possibly be removed.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/linux/igmp.h
Simple merge

diff --cc net/ipv4/igmp.c
Simple merge