staging/lustre/lloop: Properly mark userspace pointers in lo/lloop_ioctl

When casting unsigned long userspace pointer for use with
userspace-accessing functions, need to use __user attribute to make
sparse happy.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>