tracing/user_events: Clear copied tracing state before fork duplication

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

tracing/user_events: Clear copied tracing state before fork duplication

dup_task_struct() copies user_event_mm from the parent into the child,
without grabbing a reference to it. user_event_mm_dup() should
replace it, but it leaves that copied pointer unmodified if
user_event_mm_alloc() fails.

When the child exits, user_event_mm_remove() decrements a reference
the child never owned, which ultimately frees user_event_mm, while
the parent still as a stale pointer to it. This creates a UAF, which
KASAN reports as:

BUG: KASAN: slab-use-after-free in
current_user_event_mm+0x51/0x1d0 Write of size 4 at addr
ffff888005010d30 by task init/44

Call Trace:
<TASK>
kasan_report+0xce/0x100
kasan_check_range+0x10f/0x1e0
current_user_event_mm+0x51/0x1d0
user_events_ioctl+0x82e/0x15c0
__x64_sys_ioctl+0x139/0x1c0
do_syscall_64+0xce/0x450
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Allocated by task 44:
__kasan_kmalloc+0x8f/0xa0
__kmalloc_cache_noprof+0x180/0x3a0
user_event_mm_alloc+0x3c/0x1f0
current_user_event_mm+0x88/0x1d0

Freed by task 42:
__kasan_slab_free+0x43/0x70
kfree+0x13a/0x390
process_one_work+0x696/0xf90
worker_thread+0x420/0xba0

The fix simply clears the copied pointer before any possible failure.
In case of failure, the child then has nothing to free.