ipv6 ip6_tunnel: eliminate unused recursion field from ip6_tnl{}.

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: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>