sched_ext: Avoid NULL scx_root deref through SCX_HAS_OP()

SCX_HAS_OP() tests scx_root->has_op bitmap. The bitmap is currently in a
statically allocated struct scx_sched and initialized while loading the BPF
scheduler and cleared while unloading, and thus can be tested anytime.
However, scx_root will be switched to dynamic allocation and thus won't
always be deferenceable.

Most usages of SCX_HAS_OP() are already protected by scx_enabled() either
directly or indirectly (e.g. through a task which is on SCX). However, there
are a couple places that could try to dereference NULL scx_root. Update them
so that scx_root is guaranteed to be valid before SCX_HAS_OP() is called.

- In handle_hotplug(), test whether scx_root is NULL before doing anything
  else. This is safe because scx_root updates will be protected by
  cpus_read_lock().

- In scx_tg_offline(), test scx_cgroup_enabled before invoking SCX_HAS_OP(),
  which should guarnatee that scx_root won't turn NULL. This is also in line
  with other cgroup operations. As the code path is synchronized against
  scx_cgroup_init/exit() through scx_cgroup_rwsem, this shouldn't cause any
  behavior differences.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>
Acked-by: Changwoo Min <changwoo@igalia.com>