usb: gadget: f_fs: Prevent deadlock during ep0 read loop

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

usb: gadget: f_fs: Prevent deadlock during ep0 read loop

Currently, ffs_ep0_read() holds ffs->mutex when it prepares to go to
sleep waiting for an event. When no setup events are pending, it calls
wait_event_interruptible_exclusive_locked_irq() with the mutex still
held. The wait macro deliberately drops the waitqueue spinlock before
sleeping but does not drop the mutex.

If a userspace daemon is polling ep0 via read() and the gadget is
asynchronously torn down via configfs (e.g., echo "" > UDC), a
deadlock can occur:

1. The configfs teardown calls functionfs_unbind(), which queues a
FUNCTIONFS_UNBIND event.
2. The daemon wakes up, consumes the event, and drops the mutex.
3. However, if the daemon loops and immediately issues another read()
before exiting, it reacquires ffs->mutex and again goes into an
interruptible sleep.
4. Meanwhile, functionfs_unbind() continues execution and attempts to
acquire ffs->mutex to tear down ep0req.
5. The kernel deadlocks because the configfs thread is stuck in an
uninterruptible sleep waiting for the mutex, while the userspace
daemon is in an interruptible sleep holding the mutex forever
because no more events will arrive.

To fix this, we drop both the waitqueue spinlock and ffs->mutex before
going to sleep, and use wait_event_interruptible_exclusive() instead.
Upon waking up, we jump back to the `retry` label to safely reacquire
the mutex and re-evaluate the state machine. By not sleeping with
ffs->mutex held, we natively decouple gadget teardowns (which require
the mutex) from userspace polling.