usb: gadget: f_fs: Prevent race during ffs_ep0_queue_wait

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

usb: gadget: f_fs: Prevent race during ffs_ep0_queue_wait

While performing fast composition switch, there is a possibility that the

process of ffs_ep0_write/ffs_ep0_read get into a race condition

due to ep0req being freed up from functionfs_unbind.

Consider the scenario that the ffs_ep0_write calls the ffs_ep0_queue_wait

by taking a lock &ffs->ev.waitq.lock. However, the functionfs_unbind isn't

bounded so it can go ahead and mark the ep0req to NULL, and since there

is no NULL check in ffs_ep0_queue_wait we will end up in use-after-free.

Fix this by making a serialized execution between the two functions using

a mutex_lock(ffs->mutex).