ipv6: fix inet6_lookup_listener()

A stupid refactoring bug in inet6_lookup_listener() needs to be fixed
in order to get proper SO_REUSEPORT behavior.

Fixes: 3b24d854 ("tcp/dccp: do not touch listener sk_refcnt under synflood")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>