ksmbd: fix use-after-free in kerberos authentication
In the Linux kernel, the following vulnerability has been resolved:

ksmbd: fix use-after-free in kerberos authentication

Setting sess->user = NULL was introduced to fix the dangling pointer
created by ksmbd_free_user. However, it is possible another thread could
be operating on the session and make use of sess->user after it has been
passed to ksmbd_free_user but before sess->user is set to NULL.