cdc-wdm: Fix more races on the read path

We must not allow the input buffer length to change while we're
shuffling the buffer contents.  We also mustn't clear the WDM_READ
flag after more data might have arrived.  Therefore move both of these
into the spinlocked region at the bottom of wdm_read().

When reading desc->length without holding the iuspin lock, use
ACCESS_ONCE() to ensure the compiler doesn't re-read it with
inconsistent results.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Bjørn Mork <bjorn@mork.no>
Cc: Oliver Neukum <oliver@neukum.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>