bpf: Fix deadlock between rcu_tasks_trace and event_mutex.

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

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.