net_sched: keep alloc_hash updated after hash allocation

In commit 599be01e ("net_sched: fix an OOB access in cls_tcindex")
I moved cp->hash calculation before the first
tcindex_alloc_perfect_hash(), but cp->alloc_hash is left untouched.
This difference could lead to another out of bound access.

cp->alloc_hash should always be the size allocated, we should
update it after this tcindex_alloc_perfect_hash().

Reported-and-tested-by:  <syzbot+dcc34d54d68ef7d2d53d@syzkaller.appspotmail.com>
Reported-and-tested-by:  <syzbot+c72da7b9ed57cde6fca2@syzkaller.appspotmail.com>
Fixes: 599be01e ("net_sched: fix an OOB access in cls_tcindex")
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>