smb: client: fix potential UAF in cifs_stats_proc_write()

Skip sessions that are being teared down (status == SES_EXITING) to
avoid UAF.

Cc: stable@vger.kernel.org
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/cifs_debug.c

@@ -678,6 +678,8 @@ static ssize_t cifs_stats_proc_write(struct file *file,
 			}
 #endif /* CONFIG_CIFS_STATS2 */
 			list_for_each_entry(ses, &server->smb_ses_list, smb_ses_list) {
+				if (cifs_ses_exiting(ses))
+					continue;
 				list_for_each_entry(tcon, &ses->tcon_list, tcon_list) {
 					atomic_set(&tcon->num_smbs_sent, 0);
 					spin_lock(&tcon->stat_lock);