[PATCH] USB serial ftdi_sio: Prevent userspace DoS (CVE-2006-2936)

This patch limits the amount of outstanding 'write' data that can be
queued up for the ftdi_sio driver, to prevent userspace DoS attacks (or
simple accidents) that use up all the system memory by writing lots of
data to the serial port.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>