USB: cdc-wdm: fix race leading leading to memory corruption

USB: cdc-wdm: fix race leading leading to memory corruption

commit 5c22837adca7c30b66121cf18ad3e160134268d4 upstream.

This patch fixes a race whereby a pointer to a buffer
would be overwritten while the buffer was in use leading
to a double free and a memory leak. This causes crashes.
This bug was introduced in 2.6.34

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Tested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>