ksmbd: fix WARNING "do not call blocking ops when !TASK_RUNNING"

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

ksmbd: fix WARNING "do not call blocking ops when !TASK_RUNNING"

wait_event_timeout() will set the state of the current
task to TASK_UNINTERRUPTIBLE, before doing the condition check. This
means that ksmbd_durable_scavenger_alive() will try to acquire the mutex
while already in a sleeping state. The scheduler warns us by giving
the following warning:

do not call blocking ops when !TASK_RUNNING; state=2 set at
[<0000000061515a6f>] prepare_to_wait_event+0x9f/0x6c0
WARNING: CPU: 2 PID: 4147 at kernel/sched/core.c:10099 __might_sleep+0x12f/0x160

mutex lock is not needed in ksmbd_durable_scavenger_alive().