sched_ext: Avoid NULL scx_root deref in __scx_exit()

sched_ext: Avoid NULL scx_root deref in __scx_exit()

A sched_ext scheduler may trigger __scx_exit() from a BPF timer
callback, where scx_root may not be safely dereferenced.

This can lead to a NULL pointer dereference as shown below (triggered by
scx_tickless):

 BUG: kernel NULL pointer dereference, address: 0000000000000330
...
 CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.14.0-virtme #1 PREEMPT(full)
 RIP: 0010:__scx_exit+0x2b/0x190
...
 Call Trace:
  <IRQ>
  scx_bpf_get_idle_smtmask+0x59/0x80
  bpf_prog_8320d4217989178c_dispatch_all_cpus+0x35/0x1b6
...
  bpf_prog_97f847d871513f95_sched_timerfn+0x4c/0x264
  bpf_timer_cb+0x7a/0x140
  __hrtimer_run_queues+0x1f9/0x3a0
  hrtimer_run_softirq+0x8c/0xd0
  handle_softirqs+0xd3/0x3d0
  __irq_exit_rcu+0x9a/0xc0
  irq_exit_rcu+0xe/0x20

Fix this by checking for a valid scx_root and adding proper RCU
protection.

Fixes: 48e1267773866 ("sched_ext: Introduce scx_sched")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>