ksmbd: fix use-after-free in kerberos authentication

A use-after-free vulnerability in Linux kernel's ksmbd Kerberos authentication allows a thread to access freed memory when another thread clears the session user pointer, potentially causing a crash or system compromise.

The vulnerability occurs in ksmbd's Kerberos authentication path where sess->user is freed via ksmbd_free_user but a race condition permits concurrent threads to dereference the freed pointer before it is nullified, leading to use-after-free memory access. This multi-threaded race condition requires no special privileges and can be triggered remotely during Kerberos authentication handshake, resulting in denial of service or potential code execution.