usb: gadget: ffs: race between ffs_epfile_io() and ffs_func_eps_disable()
ffs_epfile_io() is called from userspace, while ffs_func_eps_disable() might be
called from USB disconnect interrupt, the two functions would run in parallel
but they are not well protected, that epfile->ep would be removed by
ffs_func_eps_disable() during ffs_epfile_io() is referring this pointer, then
it leads to kernel PANIC.