ipv6: fix inet6_csk_update_pmtu() return value

In case of error, inet6_csk_update_pmtu() should consistently
return NULL.

Bug added in commit 35ad9b9c
(ipv6: Add helper inet6_csk_update_pmtu().)

Reported-by: Lluís Batlle i Rossell <viric@viric.name>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>