sched: act_ctinfo: use extack error reporting

Use extack error reporting mechanism in addition to returning -EINVAL

NL_SET_ERR_* code shamelessy copy/paste/adjusted from act_pedit &
sch_cake and used as reference as to what I should have done in the
first place.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>