tty: n_gsm: fix mux activation issues in gsm_config()

The current implementation activates the mux if it was restarted and opens
the control channel if the mux was previously closed and we are now acting
as initiator instead of responder, which is the default setting.
This has two issues.
1) No mux is activated if we keep all default values and only switch to
initiator. The control channel is not allocated but will be opened next
which results in a NULL pointer dereference.
2) Switching the configuration after it was once configured while keeping
the initiator value the same will not reopen the control channel if it was
closed due to parameter incompatibilities. The mux remains dead.

Fix 1) by always activating the mux if it is dead after configuration.
Fix 2) by always opening the control channel after mux activation.

Fixes: e1eaea4 ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220504081733.3494-2-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -2352,6 +2352,7 @@ static void gsm_copy_config_values(struct gsm_mux *gsm,
 
 static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
 {
+	int ret = 0;
 	int need_close = 0;
 	int need_restart = 0;
 
@@ -2419,10 +2420,13 @@ static int gsm_config(struct gsm_mux *gsm, struct gsm_config *c)
 	 * FIXME: We need to separate activation/deactivation from adding
 	 * and removing from the mux array
 	 */
-	if (need_restart)
-		gsm_activate_mux(gsm);
-	if (gsm->initiator && need_close)
-		gsm_dlci_begin_open(gsm->dlci[0]);
+	if (gsm->dead) {
+		ret = gsm_activate_mux(gsm);
+		if (ret)
+			return ret;
+		if (gsm->initiator)
+			gsm_dlci_begin_open(gsm->dlci[0]);
+	}
 	return 0;
 }