[DCCP]: Initialise write_xmit_timer also on passive sockets

The TX CCID needs the write_xmit_timer for delaying packet sends. Previously
this timer was only activated on active (connecting) sockets.

This patch initialises the write_xmit_timer in sync with the other timers, i.e.
the timer will be ready on any socket. This is used by applications with a
listening socket which start to stream after receiving an initiation by the
client.  The write_xmit_timer is stopped when the application closes, as before.

Was tested to work and to remove the timer bug reported on dccp@vger.

Also moved timer initialisation into timer.c (static). 

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>