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>