cifs: fix session state check in reconnect to avoid use-after-free issue

Don't collect exiting session in smb2_reconnect_server(), because it
will be released soon.

Note that the exiting session will stay in server->smb_ses_list until
it complete the cifs_free_ipc() and logoff() and then delete itself
from the list.
Signed-off-by: Winston Wen <wentao@uniontech.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/smb2pdu.c

@@ -3797,6 +3797,12 @@ void smb2_reconnect_server(struct work_struct *work)
 
 	spin_lock(&cifs_tcp_ses_lock);
 	list_for_each_entry(ses, &pserver->smb_ses_list, smb_ses_list) {
+		spin_lock(&ses->ses_lock);
+		if (ses->ses_status == SES_EXITING) {
+			spin_unlock(&ses->ses_lock);
+			continue;
+		}
+		spin_unlock(&ses->ses_lock);
 
 		tcon_selected = false;