usb: class: cdc-wdm: fix reordering issue in read code path

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

usb: class: cdc-wdm: fix reordering issue in read code path

Quoting the bug report:

Due to compiler optimization or CPU out-of-order execution, the

desc->length update can be reordered before the memmove. If this

happens, wdm_read() can see the new length and call copy_to_user() on

uninitialized memory. This also violates LKMM data race rules [1].

Fix it by using WRITE_ONCE and memory barriers.