tracing: Do not initialize tracefs work if tracing is disabled

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

tracing: Do not initialize tracefs work if tracing is disabled

If trace buffer allocation fails during early boot (e.g. in
early_trace_init() -> allocate_trace_buffers()), tracing_disabled is
left set to 1 and the global trace array events list remains
uninitialized.

Later during boot, tracer_init_tracefs() (invoked via fs_initcall)
queues tracerfs_init_work asynchronously without checking if tracing was
disabled. When tracer_init_tracefs_work_func() runs, it calls
event_trace_init() and init_tracer_tracefs(&global_trace, NULL)
unconditionally. This leads to __find_event_file() dereferencing an
uninitialized tr->events list, causing an early boot NULL pointer
dereference Oops:

BUG: Kernel NULL pointer dereference on read at 0x00000010
Faulting instruction address: 0xc000000000485be0
Oops: Kernel access of bad area, sig: 7 [#1]
...
NIP [c000000000485be0] __find_event_file+0x70/0x3c0
LR [c000000000448874] init_tracer_tracefs+0x274/0xc80
Call Trace:
init_tracer_tracefs+0x274/0xc80
tracer_init_tracefs_work_func+0x50/0x320
process_one_work+0x1e8/0x5c0
worker_thread+0x1dc/0x3d0
kthread+0x194/0x1b0
start_kernel_thread+0x14/0x18

Check tracing_disabled at the beginning of tracer_init_tracefs_work_func()
and return immediately if tracing is disabled or failed to initialize,
preventing the crash.