ipv6: Unshare ip6_nd_hdr() and change return type to void.

- move ip6_nd_hdr() to its users' source files.
  In net/ipv6/mcast.c, it will be called ip6_mc_hdr().
- make return type to void since this function never fails.

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