sched_ext: Fix ops->priv clobber on concurrent attach/detach

sched_ext: Fix ops->priv clobber on concurrent attach/detach

Under heavy concurrent attach/detach operations, scx_claim_exit() can
trigger a NULL pointer dereference. This can be reproduced running the
reload_loop kselftests inside a virtme-ng session:

 $ vng -v -- ./tools/testing/selftests/sched_ext/runner -t reload_loop
 ...
 BUG: kernel NULL pointer dereference, address: 0000000000000400
 RIP: 0010:scx_claim_exit+0x3b/0x120
 Call Trace:
  <TASK>
  bpf_scx_unreg+0x45/0xb0
  bpf_struct_ops_map_link_dealloc+0x39/0x50
  bpf_link_release+0x18/0x20
  __fput+0x10b/0x2e0
  __x64_sys_close+0x47/0xa0

The underlying race (diagnosed by Tejun Heo) is a stomp of @ops->priv,
not a missing NULL check:

  T2 unreg(K)                       T1 reg(K)
  -----------                       ---------
  sch = ops->priv = sch_b800
  scx_disable; flush_disable_work
    [scx_root_disable: scx_root=NULL,
     mutex_unlock, state=DISABLED]
                                    mutex_lock; state ok
                                    scx_alloc_and_add_sched:
                                      ops->priv = sch_a800
                                    scx_root = sch_a800; init=0
                                    state=ENABLED; mutex_unlock
    [flush returns]
  RCU_INIT_POINTER(ops->priv, NULL) <-- clobbers sch_a800
  kobject_put(sch_b800)

T1 acquires scx_enable_mutex inside scx_root_disable()'s mutex_unlock
window and starts a fresh attach on the same kdata, assigning sch_a800
to @ops->priv. T2 then continues out of scx_disable()/flush_disable_work
and clobbers @ops->priv to NULL, leaking sch_a800; the bpf_link is gone
but state stays SCX_ENABLED, so all future attaches fail with -EBUSY
permanently. The next bpf_scx_unreg() on that kdata then reads NULL
@ops->priv and dereferences it in scx_claim_exit().

Make @ops->priv the lifecycle binding: in scx_root_enable_workfn() and
scx_sub_enable_workfn(), after the existing state check and still under
scx_enable_mutex, refuse with -EBUSY if @ops->priv is non-NULL. This
rejects an attempt to reuse a kdata that is still bound to a previous
scheduler instance, closing the race without changing the unreg side.

Fixes: 105dcd005be2 ("sched_ext: Introduce scx_prog_sched()")
Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>