sched_ext/tools: Restore backward compat with v6.12 kernels

sched_ext/tools: Restore backward compat with v6.12 kernels

Commit 111a79800aed ("tools/sched_ext: Strip compatibility macros for
cgroup and dispatch APIs") removed the compat layer for v6.12-v6.13 kfunc
renaming, but v6.12 is the current LTS kernel and will remain supported
through 2026. Restore backward compatibility so schedulers built with v6.19+
headers can run on v6.12 kernels.

The restored compat differs from the original in two ways:

1. Uses ___new/___old suffixes instead of ___compat for clarity. The new
   macros check for v6.13+ names (scx_bpf_dsq_move*), fall back to v6.12
   names (scx_bpf_dispatch_from_dsq*, scx_bpf_consume), then return safe
   no-ops for missing symbols.

2. Integrates with the args-struct-packing changes added in c0d630ba347c
   ("sched_ext: Wrap kfunc args in struct to prepare for aux__prog").
   scx_bpf_dsq_insert_vtime() now tries __scx_bpf_dsq_insert_vtime (args
   struct), then scx_bpf_dsq_insert_vtime___compat (v6.13-v6.18), then
   scx_bpf_dispatch_vtime___compat (pre-v6.13).

Forward compatibility is not restored - binaries built against v6.13 or
earlier headers won't run on v6.19+ kernels, as the old kfunc names are not
exported. This is acceptable since the priority is new binaries running on
older kernels.

Also add missing compat checks for ops.cgroup_set_bandwidth() (added v6.17)
and ops.cgroup_set_idle() (added v6.19). These need to be NULLed out in
userspace on older kernels.

Reported-by: Andrea Righi <arighi@nvidia.com>
Acked-and-tested-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>