decnet: proper socket refcounting

Better use sk_reset_timer() / sk_stop_timer() helpers to make sure we
dont access already freed/reused memory later.

Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>