Bluetooth: Avoid rfcomm_session_timeout using freed session

Use del_timer_sync() instead of del_timer() as this ensures
that rfcomm_session_timeout() is not running on a different
CPU when rfcomm_session_put() is called. This avoids a race
condition on SMP systems because potentially
rfcomm_session_timeout() could reuse the freed RFCOMM session
structure caused by the execution of rfcomm_session_put().

Note that this modification makes the reason for the RFCOMM
session refcnt mechanism redundant.
Signed-off-by: Dean Jenkins <Dean_Jenkins@mentor.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>

net/bluetooth/rfcomm/core.c

@@ -257,7 +257,7 @@ static void rfcomm_session_clear_timer(struct rfcomm_session *s)
 {
 	BT_DBG("session %p state %ld", s, s->state);
 
-	if (del_timer(&s->timer))
+	if (del_timer_sync(&s->timer))
 		rfcomm_session_put(s);
 }