[IPV6]: Clean up hop-by-hop options handler.

- Removed unused argument (nhoff) for ipv6_parse_hopopts().
- Make ipv6_parse_hopopts() to align with other extension header
  handlers.
- Removed pointless assignment (hdr), which is not used afterwards.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>