Fix "tunnels: Don't apply GRO to multiple layers of encapsulation."

Commit dd4fff23 which is commit
fac8e0f5 upstream, seems to have
included the sit_gro_receive function, yet it never is used, causing an
obvious warning message.  Hook it up to the correct sit_offload
structure.

Note, for 3.16, the backport of fac8e0f5
does not include this function, nor the ipip case.  I'm guessing that
this is not correct for 3.18, as one of the functions was included, but
could be totally wrong.

Cc: Jesse Gross <jesse@kernel.org>
Cc: David S. Miller <davem@davemloft.net>
Cc: Ben Hutchings <ben@decadent.org.uk>
Cc: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

net/ipv6/ip6_offload.c

@@ -305,7 +305,7 @@ static struct packet_offload ipv6_packet_offload __read_mostly = {
 static const struct net_offload sit_offload = {
 	.callbacks = {
 		.gso_segment	= ipv6_gso_segment,
-		.gro_receive	= ipv6_gro_receive,
+		.gro_receive	= sit_gro_receive,
 		.gro_complete	= ipv6_gro_complete,
 	},
 };