vxlan: leave multicast group when vxlan device down

vxlan_group_used only allows device to leave multicast group
when the remote_ip of this vxlan device is difference from
other vxlan devices' remote_ip. this will cause device not
leave multicast group untile the vn_sock of this vxlan deivce
being released.

The check in vxlan_group_used is not quite precise. since even
the remote_ip is same, but these vxlan devices may use different
lower devices, and they may use different vn_socks.

Only when some vxlan devices use the same vn_sock,same lower
device and same remote_ip, the mc_list of the vn_sock should
not be changed.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>