ipv6: help __ip6_finish_output() inlining

There are two callers of __ip6_finish_output(), both are in
ip6_finish_output(). We can combine the call sites into one and handle
return code after, that will inline __ip6_finish_output().

Note, error handling under NET_XMIT_CN will only return 0 if
__ip6_finish_output() succeded, and in this case it return 0.
Considering that NET_XMIT_SUCCESS is 0, it'll be returning exactly the
same result for it as before.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>