staging: lustre: potential underflow in mdc_iocontrol()

Smatch complains that "data->ioc_plen2" is a user controlled value and,
since we cast to signed int, the limit check can underflow.  It's not
very serious because probably the copy_to_user() would return -EFAULT
on every arch that matters instead of creating an info leak.  Also I
haven't followed it through to see if the value is really user
controlled.

But definitely it would be safer to cast to unsigned so let's do that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>