bpf: Fix deadlock between rcu_tasks_trace and event_mutex.

Fix the following deadlock:
CPU A
_free_event()
perf_kprobe_destroy()
mutex_lock(&event_mutex)
perf_trace_event_unreg()
synchronize_rcu_tasks_trace()

There are several paths where _free_event() grabs event_mutex
and calls sync_rcu_tasks_trace. Above is one such case.

CPU B
bpf_prog_test_run_syscall()
rcu_read_lock_trace()
bpf_prog_run_pin_on_cpu()
bpf_prog_load()
bpf_tracing_func_proto()
trace_set_clr_event()
mutex_lock(&event_mutex)

Delegate trace_set_clr_event() to workqueue to avoid
such lock dependency.

Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/trace/bpf_trace.c

···
 	.arg2_type	= ARG_CONST_SIZE,
 };
 
-static void __set_printk_clr_event(void)
+static void __set_printk_clr_event(struct work_struct *work)
 {
 	/*
 	 * This program might be calling bpf_trace_printk,
···
 	if (trace_set_clr_event("bpf_trace", "bpf_trace_printk", 1))
 		pr_warn_ratelimited("could not enable bpf_trace_printk events");
 }
+static DECLARE_WORK(set_printk_work, __set_printk_clr_event);
 
 const struct bpf_func_proto *bpf_get_trace_printk_proto(void)
 {
-	__set_printk_clr_event();
+	schedule_work(&set_printk_work);
 	return &bpf_trace_printk_proto;
 }
 
···
 
 const struct bpf_func_proto *bpf_get_trace_vprintk_proto(void)
 {
-	__set_printk_clr_event();
+	schedule_work(&set_printk_work);
 	return &bpf_trace_vprintk_proto;
 }