net: sched: use kvmalloc() for class hash tables

High order GFP_KERNEL allocations can stress the host badly.

Use modern kvmalloc_array()/kvfree() instead of custom
allocations.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>