[PATCH RESEND net-next] ip_tunnel_core: Fix build for archs without _HAVE_ARCH_IPV6_CSUM

[Stefano Brivio <sbrivio@redhat.com>]

On architectures defining _HAVE_ARCH_IPV6_CSUM, we get
csum_ipv6_magic() defined by means of arch checksum.h headers. On
other architectures, we actually need to include net/ip6_checksum.h
to be able to use it.

Without this include, building with defconfig breaks at least for
s390.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 4cb47a8644cc ("tunnels: PMTU discovery support for directly bridged IP packets")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
I'm submitting this for net-next despite the fact it's closed, as
the offending code isn't merged to net.git yet. Should I rather
submit this to... linux-next?

Re-sending as original copy seems to be stuck in some SMTP queue.

 net/ipv4/ip_tunnel_core.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index 9ddee2a0c66d..75c6013ff9a4 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -25,6 +25,7 @@
 #include <net/protocol.h>
 #include <net/ip_tunnels.h>
 #include <net/ip6_tunnel.h>
+#include <net/ip6_checksum.h>
 #include <net/arp.h>
 #include <net/checksum.h>
 #include <net/dsfield.h>
-- 
2.27.0