net_sched: sch_skbprio: add message validation to skbprio_change()

Do not assume the attribute has the right size.

Fixes: aea5f654 ("net/sched: add skbprio scheduler")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>