ipv6: fix exthdrs offload registration in out_rt path

We previously register IPPROTO_ROUTING offload under inet6_add_offload(),
but in error path, we try to unregister it with inet_del_offload(). This
doesn't seem correct, it should actually be inet6_del_offload(), also
ipv6_exthdrs_offload_exit() from that commit seems rather incorrect (it
also uses rthdr_offload twice), but it got removed entirely later on.

Fixes: 3336288a ("ipv6: Switch to using new offload infrastructure.")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>