ipv6: partially inline ipv6_fixup_options

Inline a part of ipv6_fixup_options() to avoid extra overhead on
function call if opt is NULL.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>