net: sched: gred: use extack to provide more details on configuration errors

Add extack messages to -EINVAL errors, to help users identify
their mistakes.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>