Commit a43912ab... ("tunnel: eliminate recursion field") eliminated
use of recursion field from tunnel structures, but its definition
still exists in ip6_tnl{}.

Let's remove that unused field.
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>