ksmbd: fix use-after-free in SMB request handling

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

ksmbd: fix use-after-free in SMB request handling

A race condition exists between SMB request handling in
`ksmbd_conn_handler_loop()` and the freeing of `ksmbd_conn` in the
workqueue handler `handle_ksmbd_work()`. This leads to a UAF.
- KASAN: slab-use-after-free Read in handle_ksmbd_work
- KASAN: slab-use-after-free in rtlock_slowlock_locked

This race condition arises as follows:
- `ksmbd_conn_handler_loop()` waits for `conn->r_count` to reach zero:
`wait_event(conn->r_count_q, atomic_read(&conn->r_count) == 0);`
- Meanwhile, `handle_ksmbd_work()` decrements `conn->r_count` using
`atomic_dec_return(&conn->r_count)`, and if it reaches zero, calls
`ksmbd_conn_free()`, which frees `conn`.
- However, after `handle_ksmbd_work()` decrements `conn->r_count`,
it may still access `conn->r_count_q` in the following line:
`waitqueue_active(&conn->r_count_q)` or `wake_up(&conn->r_count_q)`
This results in a UAF, as `conn` has already been freed.

The discovery of this UAF can be referenced in the following PR for
syzkaller's support for SMB requests.