vxlan: fix igmp races

There are two race conditions in existing code for doing IGMP
management in workqueue in vxlan. First, the vxlan_group_used
function checks the list of vxlan's without any protection, and
it is possible for open followed by close to occur before the
igmp work queue runs.

To solve these move the check into vxlan_open/stop so it is
protected by RTNL. And split into two work structures so that
there is no racy reference to underlying device state.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>