BACKPORT: sched_ext: Use SCX_TASK_READY test instead of tryget_task_struct() during class switch

ddf7233fcab6 ("sched/ext: Fix invalid task state transitions on class
switch") added tryget_task_struct() test during scx_enable()'s class
switching loop. The reason for the addition was to avoid enabling tasks
which skipped prep in the previous loop due to being dead.

While tryget_task_struct() does work for this purpose as tasks that fail
tryget always will fail it, it's a bit roundabout. A more direct way is
testing whether the task is in READY state. Switch to testing
SCX_TASK_READY directly.

BackPort Reason: This upstream patch can fixes a potential deadlock
issue during class switch within scx_ops_enable().

During class switch, scx increments task's usage via
tryget_task_struct(), and later decrements it via put_task_struct(). If
the task is already in TASK_DEAD state, there is a risk that scx could
bring the usage down to zero, triggering __put_task_struct(). This
sequence may lead to a deadlock through the following chain:

```
static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
{
    percpu_down_write(&scx_fork_rwsem);

    // Step1: hold scx_tasks_lock
    scx_task_iter_start(&sti);

    // Step2: hold rq lock & pi lock
    while ((p = scx_task_iter_next_locked(&sti))) {

        // p->usage is 1 but p is exiting.
        if (!tryget_task_struct(p))
            continue;

        // doing class switch

        // Step3:
        //  If p->usage become zero, __put_task_struct() will be called,
        //  and then acquire scx_tasks_lock again in sched_ext_free().
        put_task_struct(p);
    }
    scx_task_iter_stop(&sti);
    percpu_up_write(&scx_fork_rwsem);
}
```

CallStack:
        vmlinux          queued_spin_lock_slowpath() + 140
        vmlinux          _raw_spin_lock_irqsave() + 144
        vmlinux          sched_ext_free() + 44
        vmlinux          __put_task_struct() + 56
        vmlinux          bpf_scx_reg() + 2256

Bug: 479683877
Change-Id: I340f514d97eba7c628982a39ebb5e6b351704759
Signed-off-by: Tejun Heo <tj@kernel.org>
(cherry picked from commit b7d4b28db7da89de1d03316c4e67b5a47390fcd3)
[xieliujie: Resolved minor conflict in kernel/sched/ext.c ]
Signed-off-by: xieliujie <xieliujie@xiaomi.corp-partner.google.com>