net: make ip6tunnel_xmit definition conditional

Moving the caller of iptunnel_xmit_stats causes a build error in
randconfig builds that disable CONFIG_INET:

In file included from ../net/xfrm/xfrm_input.c:17:0:
../include/net/ip6_tunnel.h: In function 'ip6tunnel_xmit':
../include/net/ip6_tunnel.h:93:2: error: implicit declaration of function 'iptunnel_xmit_stats' [-Werror=implicit-function-declaration]
  iptunnel_xmit_stats(dev, pkt_len);

The reason is that the iptunnel_xmit_stats definition is hidden
inside #ifdef CONFIG_INET but the caller is not. We can change
one or the other to fix it, and this patch adds a second #ifdef
around ip6tunnel_xmit() to avoid seeing the invalid call.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 039f5062

 ("ip_tunnel: Move stats update to iptunnel_xmit()")
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>