ip6: move ipip6_err_gen_icmpv6_unreach()

We want to use this helper from GRE as well, so this is
the time to move it in net/ipv6/icmp.c

Also add a @nhs parameter, since SIT and GRE have different
values for the header(s) to skip.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>