vxlan: use __dev_get_by_index instead of dev_get_by_index to find interface

The following call chains indicate that vxlan_fdb_parse() is
under rtnl_lock protection. So if we use __dev_get_by_index()
instead of dev_get_by_index() to find interface handler in it,
this would help us avoid to change interface reference counter.

rtnetlink_rcv()
  rtnl_lock()
  netlink_rcv_skb()
    rtnl_fdb_add()
      vxlan_fdb_add()
        vxlan_fdb_parse()
  rtnl_unlock()

rtnetlink_rcv()
  rtnl_lock()
  netlink_rcv_skb()
    rtnl_fdb_del()
      vxlan_fdb_del()
        vxlan_fdb_parse()
  rtnl_unlock()

Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>