ksmbd: fix null pointer dereference in destroy_previous_session

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: fix null pointer dereference in destroy_previous_session

If client set ->PreviousSessionId on kerberos session setup stage,

NULL pointer dereference error will happen. Since sess->user is not

set yet, It can pass the user argument as NULL to destroy_previous_session.

sess->user will be set in ksmbd_krb5_authenticate(). So this patch move

calling destroy_previous_session() after ksmbd_krb5_authenticate().