sched_ext: Set errno on ENABLING -> ENABLED transition failure

If the SCX_ENABLING -> SCX_ENABLED cmpxchg at the tail of
scx_root_enable_workfn() fails, the function jumps to err_disable
without setting ret. At that point ret still holds the return value
of the last successful __scx_init_task() call, which is 0, so the
err_disable fallback reports the meaningless message:

  scx_root_enable() failed (0)

Set ret = -EBUSY, consistent with the other enable-state guards at
the top of the same function, so the fallback always reports a real
errno.

Signed-off-by: Liang Luo <luoliang@kylinos.cn>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/sched/ext/ext.c

@@ -7520,6 +7520,7 @@ static void scx_root_enable_workfn(struct kthread_work *work)
 
 	if (!scx_tryset_enable_state(SCX_ENABLED, SCX_ENABLING)) {
 		WARN_ON_ONCE(atomic_read(&sch->exit_kind) == SCX_EXIT_NONE);
+		ret = -EBUSY;
 		goto err_disable;
 	}