net_sched: minor netns related cleanup

These changes were suggested by Alexey Dobriyan <adobriyan@gmail.com>:

  - psched_show() does not use any private data so just pass NULL to
    psched_open()

  - remove unnecessary return statement

Signed-off-by: Tom Goff <thomas.goff@boeing.com>
Signed-off-by: David S. Miller <davem@davemloft.net>