pkt_sched: Add multiqueue handling to qdisc_graft().

Move the destruction of the old queue into qdisc_graft().

When operating on a root qdisc (ie. "parent == NULL"), apply
the operation to all queues.  The caller has grabbed a single
implicit reference for this graft, therefore when we apply the
change to more than one queue we must grab additional qdisc
references.

Otherwise, we are operating on a class of a specific parent qdisc, and
therefore no multiqueue handling is necessary.

Signed-off-by: David S. Miller <davem@davemloft.net>