sched_ext: Fix NULL dereference in scx_bpf_cpu_rq() warning

sched_ext: Fix NULL dereference in scx_bpf_cpu_rq() warning

When printing the deprecation warning for scx_bpf_cpu_rq(), we may hit a
NULL pointer dereference if the kfunc is called before a BPF scheduler
is fully attached, for example, when invoked from a BPF timer or during
ops.init():

 [   50.752775] BUG: kernel NULL pointer dereference, address: 0000000000000331
 ...
 [   50.764205] RIP: 0010:scx_bpf_cpu_rq+0x30/0xa0
 ...
 [   50.787661] Call Trace:
 [   50.788398]  <TASK>
 [   50.789061]  bpf_prog_08f7fd2dcb187aaf_wakeup_timerfn+0x75/0x1a8
 [   50.792477]  bpf_timer_cb+0x7e/0x140
 [   50.796003]  hrtimer_run_softirq+0x91/0xe0
 [   50.796952]  handle_softirqs+0xce/0x3c0
 [   50.799087]  run_ksoftirqd+0x3e/0x70
 [   50.800197]  smpboot_thread_fn+0x133/0x290
 [   50.802320]  kthread+0x115/0x220
 [   50.804984]  ret_from_fork+0x17a/0x1d0
 [   50.806920]  ret_from_fork_asm+0x1a/0x30
 [   50.807799]  </TASK>

Fix this by only printing the warning once the scheduler is fully
registered.

Fixes: 5c48d88fe0049 ("sched_ext: deprecation warn for scx_bpf_cpu_rq()")
Cc: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>