tcp/dccp: no longer use twsk_net(tw) from tw_timer_handler()

We will soon get rid of inet_twsk_purge().

This means that tw_timer_handler() might fire after
a netns has been dismantled/freed.

Instead of adding a function (and data structure) to find a netns
from tw->tw_net_cookie, just update the SNMP counters
a bit earlier, when the netns is known to be alive.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>