smb: client: fix potential UAF in cifs_stats_proc_show()

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

Cc: [e-mail]
Signed-off-by: Paulo Alcantara (Red Hat) <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/smb/client/cifs_debug.c

···
 			}
 #endif /* 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) {
 				i++;
 				seq_printf(m, "\n%d) %s", i, tcon->tree_name);