net_sched: prio: properly report out of memory errors

At Qdisc creation or change time, prio_tune() creates missing
pfifo qdiscs but does not return an error code if one
qdisc could not be allocated.

Leaving a qdisc in non operational state without telling user
anything about this problem is not good.

Also, testing if we replace something different than noop_qdisc
a second time makes no sense so I removed useless code.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>