tracing: fprobe: fix suspicious rcu usage in fprobe_entry

rcu_read_lock() is not needed in fprobe_entry, but rcu_dereference_check()
is used in rhltable_lookup(), which causes suspicious RCU usage warning:

  WARNING: suspicious RCU usage
  6.17.0-rc1-00001-gdfe0d675df82 #1 Tainted: G S
  -----------------------------
  include/linux/rhashtable.h:602 suspicious rcu_dereference_check() usage!
  ......
  stack backtrace:
  CPU: 1 UID: 0 PID: 4652 Comm: ftracetest Tainted: G S
  Tainted: [S]=CPU_OUT_OF_SPEC, [I]=FIRMWARE_WORKAROUND
  Hardware name: Dell Inc. OptiPlex 7040/0Y7WYT, BIOS 1.1.1 10/07/2015
  Call Trace:
   <TASK>
   dump_stack_lvl+0x7c/0x90
   lockdep_rcu_suspicious+0x14f/0x1c0
   __rhashtable_lookup+0x1e0/0x260
   ? __pfx_kernel_clone+0x10/0x10
   fprobe_entry+0x9a/0x450
   ? __lock_acquire+0x6b0/0xca0
   ? find_held_lock+0x2b/0x80
   ? __pfx_fprobe_entry+0x10/0x10
   ? __pfx_kernel_clone+0x10/0x10
   ? lock_acquire+0x14c/0x2d0
   ? __might_fault+0x74/0xc0
   function_graph_enter_regs+0x2a0/0x550
   ? __do_sys_clone+0xb5/0x100
   ? __pfx_function_graph_enter_regs+0x10/0x10
   ? _copy_to_user+0x58/0x70
   ? __pfx_kernel_clone+0x10/0x10
   ? __x64_sys_rt_sigprocmask+0x114/0x180
   ? __pfx___x64_sys_rt_sigprocmask+0x10/0x10
   ? __pfx_kernel_clone+0x10/0x10
   ftrace_graph_func+0x87/0xb0

As we discussed in [1], fix this by using guard(rcu)() in fprobe_entry()
to protect the rhltable_lookup() and rhl_for_each_entry_rcu() with
rcu_read_lock and suppress this warning.

Link: https://lore.kernel.org/all/20250904062729.151931-1-dongml2@chinatelecom.cn/

Link: https://lore.kernel.org/all/20250829021436.19982-1-dongml2@chinatelecom.cn/ [1]
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202508281655.54c87330-lkp@intel.com
Fixes: dfe0d675df82 ("tracing: fprobe: use rhltable for fprobe_ip_table")
Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

kernel/trace/fprobe.c

@@ -269,6 +269,7 @@ static int fprobe_entry(struct ftrace_graph_ent *trace, struct fgraph_ops *gops,
 	if (WARN_ON_ONCE(!fregs))
 		return 0;
 
+	guard(rcu)();
 	head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
 	reserved_words = 0;
 	rhl_for_each_entry_rcu(node, pos, head, hlist) {