ip6tnl: less dev_put() calls

Using dev_get_by_index_rcu() in ip6_tnl_rcv_ctl() & ip6_tnl_xmit_ctl()
avoids touching device refcount.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>