net: use DEBUG_NET_WARN_ON_ONCE() in __release_sock()

Check against skb dst in socket backlog has never triggered
in past years.

Keep the check omly for CONFIG_DEBUG_NET=y builds.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>