net_sched: fix class grafting errno codes

If the parent qdisc doesn't support classes, use EOPNOTSUPP.
If the parent class doesn't exist, use ENOENT. Currently EINVAL
is returned in both cases.

Additionally check whether grafting is supported and remove a now
unnecessary graft function from sch_ingress.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>