tty: n_gsm: fix wrong DLCI release order

jira VULN-676
cve-pre CVE-2023-6546
commit-author Daniel Starke <daniel.starke@siemens.com>
commit deefc58

The current DLCI release order starts with the control channel followed by
the user channels. Reverse this order to keep the control channel open
until all user channels have been released.

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/20220414094225.4527-9-daniel.starke@siemens.com
	Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit deefc58)
	Signed-off-by: David Gomez <dgomez@ciq.com>

Author: dvdgomez

drivers/tty/n_gsm.c

@@ -2079,8 +2079,8 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm, bool disc)
 	if (i == MAX_MUX)
 		return;
 
-	/* Free up any link layer users */
-	for (i = 0; i < NUM_DLCI; i++)
+	/* Free up any link layer users and finally the control channel */
+	for (i = NUM_DLCI - 1; i >= 0; i--)
 		if (gsm->dlci[i])
 			gsm_dlci_release(gsm->dlci[i]);
 	mutex_unlock(&gsm->mutex);