tracing: Fix warning in trace_buffered_event_disable()

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

tracing: Fix warning in trace_buffered_event_disable()

Warning happened in trace_buffered_event_disable() at

WARN_ON_ONCE(!trace_buffered_event_ref)

Call Trace:

? __warn+0xa5/0x1b0

? trace_buffered_event_disable+0x189/0x1b0

__ftrace_event_enable_disable+0x19e/0x3e0

free_probe_data+0x3b/0xa0

unregister_ftrace_function_probe_func+0x6b8/0x800

event_enable_func+0x2f0/0x3d0

ftrace_process_regex.isra.0+0x12d/0x1b0

ftrace_filter_write+0xe6/0x140

vfs_write+0x1c9/0x6f0

[...]

The cause of the warning is in __ftrace_event_enable_disable(),

trace_buffered_event_enable() was called once while

trace_buffered_event_disable() was called twice.

Reproduction script show as below, for analysis, see the comments:

```

#!/bin/bash

cd /sys/kernel/tracing/

# 1. Register a 'disable_event' command, then:

# 1) SOFT_DISABLED_BIT was set;

# 2) trace_buffered_event_enable() was called first time;

echo 'cmdline_proc_show:disable_event:initcall:initcall_finish' > \

set_ftrace_filter

# 2. Enable the event registered, then:

# 1) SOFT_DISABLED_BIT was cleared;

# 2) trace_buffered_event_disable() was called first time;

echo 1 > events/initcall/initcall_finish/enable

# 3. Try to call into cmdline_proc_show(), then SOFT_DISABLED_BIT was

# set again!!!

cat /proc/cmdline

# 4. Unregister the 'disable_event' command, then:

# 1) SOFT_DISABLED_BIT was cleared again;

# 2) trace_buffered_event_disable() was called second time!!!

echo '!cmdline_proc_show:disable_event:initcall:initcall_finish' > \

set_ftrace_filter

```

To fix it, IIUC, we can change to call trace_buffered_event_enable() at

fist time soft-mode enabled, and call trace_buffered_event_disable() at

last time soft-mode disabled.