vxlan: remove vxlan_group_used in vxlan_open

In vxlan_open, vxlan_group_used always returns true,
because the state of the vxlan deivces which we want
to open has alreay been running. and it has already
in vxlan_list.

Since ip_mc_join_group takes care of the reference
of struct ip_mc_list. removing vxlan_group_used here
is safe.

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