net/sched: cls_u32: Replace one-element array with flexible-array member

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of a flexible-array member in
struct tc_u_hnode and use the struct_size() helper to calculate the
size for the allocations. Commit 5778d39d

 ("net_sched: fix struct
tc_u_hnode layout in u32") makes it clear that the code is expected to
dynamically allocate divisor + 1 entries for ->ht[] in tc_uhnode. Also,
based on other observations, as the piece of code below:

1232                 for (h = 0; h <= ht->divisor; h++) {
1233                         for (n = rtnl_dereference(ht->ht[h]);
1234                              n;
1235                              n = rtnl_dereference(n->next)) {
1236                                 if (tc_skip_hw(n->flags))
1237                                         continue;
1238
1239                                 err = u32_reoffload_knode(tp, n, add, cb,
1240                                                           cb_priv, extack);
1241                                 if (err)
1242                                         return err;
1243                         }
1244                 }

we can assume that, in general, the code is actually expecting to allocate
that extra space for the one-element array in tc_uhnode, everytime it
allocates memory for instances of tc_uhnode or tc_u_common structures.
That's the reason for passing '1' as the last argument for struct_size()
in the allocation for _root_ht_ and _tp_c_, and 'divisor + 1' in the
allocation code for _ht_.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays

Tested-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/5f7062af.z3T9tn9yIPv6h5Ny%25lkp@intel.com/
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>