smb: client: fix potential UAF in cifs_stats_proc_show()

commit 0865ffef upstream.

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>
[ cifs_debug.c was moved from fs/cifs to fs/smb/client since
  38c8a9a5 ("smb: move client and server files to common directory fs/smb").
  The cifs_ses_exiting() was introduced to cifs_debug.c since
  ca545b7f ("smb: client: fix potential UAF in cifs_debug_files_proc_show()")
  which has been sent to upstream already. ]
Signed-off-by: Jianqi Ren <jianqi.ren.cn@windriver.com>
Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>