net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT_RT=y

net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT_RT=y

syzbot reported a WARNING in est_timer() [1]

Problem here is that with CONFIG_PREEMPT_RT=y, timer callbacks
can be preempted.

Adopt preempt_disable_nested()/preempt_enable_nested() to fix this.

[1]
 WARNING: CPU: 0 PID: 16 at ./include/linux/seqlock.h:221 __seqprop_assert include/linux/seqlock.h:221 [inline]
 WARNING: CPU: 0 PID: 16 at ./include/linux/seqlock.h:221 est_timer+0x6dc/0x9f0 net/core/gen_estimator.c:93
Modules linked in:
CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
 RIP: 0010:__seqprop_assert include/linux/seqlock.h:221 [inline]
 RIP: 0010:est_timer+0x6dc/0x9f0 net/core/gen_estimator.c:93
Call Trace:
 <TASK>
  call_timer_fn+0x17e/0x5f0 kernel/time/timer.c:1747
  expire_timers kernel/time/timer.c:1798 [inline]
  __run_timers kernel/time/timer.c:2372 [inline]
  __run_timer_base+0x648/0x970 kernel/time/timer.c:2384
  run_timer_base kernel/time/timer.c:2393 [inline]
  run_timer_softirq+0xb7/0x180 kernel/time/timer.c:2403
  handle_softirqs+0x22c/0x710 kernel/softirq.c:579
  __do_softirq kernel/softirq.c:613 [inline]
  run_ktimerd+0xcf/0x190 kernel/softirq.c:1043
  smpboot_thread_fn+0x53f/0xa60 kernel/smpboot.c:160
  kthread+0x70e/0x8a0 kernel/kthread.c:463
  ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
  ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>

Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.")
Reported-by: syzbot+72db9ee39db57c3fecc5@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/68adf6fa.a70a0220.3cafd4.0000.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20250827162352.3960779-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>