net/x25: handle call collisions

If a session in X25_STATE_1 (Awaiting Call Accept) receives a call
request, the session will be closed (x25_disconnect), cause=0x01
(Number Busy) and diag=0x48 (Call Collision) will be set and a clear
request will be send.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: David S. Miller <davem@davemloft.net>