staging: for dgrp, let nd_ps_desc always NUL terminated string within MAX_DESC_LEN length

Use strlcpy instead of strncpy to let nd_ps_desc always NUL terminated.

Change nd_ps_desc in nd_struct struct to MAX_DESC_LEN to avoid the
confusion (the related length checking also need be changed to
MAX_DESC_LEN - 1).

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>