smb: client: fix potential UAF in cifs_debug_files_proc_show()

commit ca545b7f 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>
[ This patch removes lock/unlock operation in routine cifs_ses_exiting()
  for ses_lock is not present in v5.10 and not ported yet. ses->status
  is protected by a global lock, cifs_tcp_ses_lock, in v5.10. ]
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>