staging: lustre: obdclass: return -EFAULT if copy_to_user() fails

We recently changed from using obd_ioctl_popdata() to calling
copy_to_user() directly.  This if statement was supposed to be deleted
but it was over looked.  "err" is zero at this point so it means we
return success.

Fixes: b03679f6 ("staging: lustre: uapi: remove obd_ioctl_popdata() wrapper")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>