tracing: Save the event file instead of the compatible histogram

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

tracing: Save the event file instead of the compatible histogram

create_field_var_hist() creates a histogram on the matched event to
supply a field variable, and records it on the target so it can be torn
down later:

hist_data = find_compatible_hist(target_hist_data, file);
...
/* Save the compatible histogram information */
var_hist->hist_data = hist_data;

hist_data is not the histogram it creates. It is one that was already
there, whose keys the new one copies. The saved pointer has a single
user, unregister_field_var_hists(), which runs when the target is
removed:

file = hist_data->field_var_hists[i]->hist_data->event_file;

find_compatible_hist() matches on keys alone, so it can pick one with no
variables of its own. check_var_refs() then returns false and the user
can remove it while the target still points at it:

BUG: KASAN: slab-use-after-free in event_hist_trigger_free+0x2b2/0x320
Read of size 8 at addr ffff8880093b90e0 by task init/1

Freed by task 1:
kfree+0x154/0x420
event_hist_trigger_free+0x1d5/0x320
event_hist_trigger_parse+0x35f3/0x69e0
trigger_process_regex+0x1a6/0x250

Save the event file instead. It is all the dereference ever wanted, and
it does not go away when the user removes a trigger.