fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching()

In the Linux kernel, the following vulnerability has been resolved:

fprobe: Use guard(rcu_sched_notrace) and check rcu_is_watching()

unregister_fprobe() and unregister_fprobe_async() (used by BPF
kprobe-multi) rely on standard RCU grace periods (synchronize_rcu()
and call_rcu()) to wait until in-flight fprobe handlers complete before
freeing the fprobe.

However, if an fprobe handler executes while RCU is not watching (such
as in the idle loop or nohz_full extended quiescent states), standard
RCU does not track preemption-disabled sections. Consequently,
synchronize_rcu() does not wait for those executions, which can lead
to a use-after-free if the fprobe is freed immediately after
unregistration. Ensure handlers exit early when !rcu_is_watching().

Furthermore, fprobe_fgraph_entry() and fprobe_ftrace_entry() previously
used guard(rcu)() and rcu_read_lock(), which invoke lockdep on every
hit under CONFIG_PROVE_LOCKING. This adds overhead and can cause lockdep
recursion if probed functions interact with lockdep.

Since rhltable_lookup() and rhl_for_each_entry_rcu() use
rcu_dereference_all_check() (which checks rcu_read_lock_any_held()),
holding preemption disabled via rcu_read_lock_sched_notrace() is fully
valid and sufficient so long as rcu_is_watching() is true.

Define and use guard(rcu_sched_notrace)() across fprobe_ftrace_entry(),
fprobe_fgraph_entry(), and fprobe_return(). This eliminates fast-path
rcu_read_lock() and lockdep overhead while guaranteeing safe grace
period synchronization.