tools/sched_ext: fix scx_show_state.py for scx_root change

tools/sched_ext: fix scx_show_state.py for scx_root change

Commit 48e126777386 ("sched_ext: Introduce scx_sched") introduced
scx_root and removed scx_ops, causing scx_show_state.py to fail when
searching for the 'scx_ops' object. [1]

Fix by using 'scx_root' instead, with NULL pointer handling.

[1]
 # drgn -s vmlinux ./tools/sched_ext/scx_show_state.py
 Traceback (most recent call last):
   File "/root/.venv/bin/drgn", line 8, in <module>
     sys.exit(_main())
              ~~~~~^^
   File "/root/.venv/lib64/python3.14/site-packages/drgn/cli.py", line 625, in _main
     runpy.run_path(
     ~~~~~~~~~~~~~~^
         script_path, init_globals={"prog": prog}, run_name="__main__"
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     )
     ^
   File "<frozen runpy>", line 287, in run_path
   File "<frozen runpy>", line 98, in _run_module_code
   File "<frozen runpy>", line 88, in _run_code
   File "./tools/sched_ext/scx_show_state.py", line 30, in <module>
     ops = prog['scx_ops']
           ~~~~^^^^^^^^^^^
 _drgn.ObjectNotFoundError: could not find 'scx_ops'

Fixes: 48e126777386 ("sched_ext: Introduce scx_sched")
Signed-off-by: Kohei Enju <enjuk@amazon.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Tejun Heo <tj@kernel.org>