ipv6: Make IP6CB(skb)->nhoff 16-bit.

Even with jumbograms I cannot see any way in which we would need
to records a larger than 65535 valued next-header offset.

The maximum extension header length is (256 << 3) == 2048.
There are only a handful of extension headers specified which
we'd even accept (say 5 or 6), therefore the largest next-header
offset we'd ever have to contend with is something less than
say 16k.

Therefore make it a u16 instead of a u32.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ipv6.h

@@ -246,7 +246,7 @@ struct inet6_skb_parm {
 	__u16			srcrt;
 	__u16			dst1;
 	__u16			lastopt;
-	__u32			nhoff;
+	__u16			nhoff;
 	__u16			flags;
 #if defined(CONFIG_IPV6_MIP6) || defined(CONFIG_IPV6_MIP6_MODULE)
 	__u16			dsthao;