nfp: abm: allocate Qdisc child table

To keep track of Qdisc hierarchy allocate a table for children
for each Qdisc.  RED Qdisc can only have one child.

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>