tracing: Take the reference before publishing the named histogram trigger

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

tracing: Take the reference before publishing the named histogram trigger

event_hist_trigger_named_init() puts the trigger on the global
named_triggers list and only then takes the reference on the trigger it
shares its histogram with:

data->ref++;

save_named_trigger(data->named_data->name, data);

ret = event_hist_trigger_init(data->named_data);
if (ret < 0) {
kfree(data->cmd_ops);
data->cmd_ops = &trigger_hist_cmd;
}

return ret;

event_hist_trigger_init() fails when alloc_hist_pad() cannot allocate, and
nothing takes the trigger back off the list on the way out.
event_hist_trigger_parse() frees it, and the next lookup by name reads the
freed object:

BUG: KASAN: slab-use-after-free in find_named_trigger+0xac/0xc0
Read of size 8 at addr ffff888009346860 by task init/1
find_named_trigger+0xac/0xc0
hist_register_trigger+0xc1/0xa00
event_hist_trigger_parse+0x3146/0x6af0
event_trigger_write+0xce/0x160
Freed by task 67:
kfree+0x154/0x420
trigger_kthread_fn+0xfd/0x160

Do the reference first and publish once it has succeeded, so that nothing
which can fail runs after the trigger becomes findable.