ksmbd: Fix user namespace mapping

ksmbd: Fix user namespace mapping

A kernel daemon should not rely on the current thread, which is unknown
and might be malicious.  Before this security fix,
ksmbd_override_fsids() didn't correctly override FS UID/GID which means
that arbitrary user space threads could trick the kernel to impersonate
arbitrary users or groups for file system access checks, leading to
file system access bypass.

This was found while investigating truncate support for Landlock:
https://lore.kernel.org/r/CAKYAXd8fpMJ7guizOjHgxEyyjoUwPsx3jLOPZP=wPYcbhkVXqA@mail.gmail.com

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: Hyunchul Lee <hyc.lee@gmail.com>
Cc: Steve French <smfrench@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20220929100447.108468-1-mic@digikod.net
Acked-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>