tracing: Have process_string() also allow arrays

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

tracing: Have process_string() also allow arrays

In order to catch a common bug where a TRACE_EVENT() TP_fast_assign()

assigns an address of an allocated string to the ring buffer and then

references it in TP_printk(), which can be executed hours later when the

string is free, the function test_event_printk() runs on all events as

they are registered to make sure there's no unwanted dereferencing.

It calls process_string() to handle cases in TP_printk() format that has

"%s". It returns whether or not the string is safe. But it can have some

false positives.

For instance, xe_bo_move() has:

TP_printk("move_lacks_source:%s, migrate object %p [size %zu] from %s to %s device_id:%s",

__entry->move_lacks_source ? "yes" : "no", __entry->bo, __entry->size,

xe_mem_type_to_name[__entry->old_placement],

xe_mem_type_to_name[__entry->new_placement], __get_str(device_id))

Where the "%s" references into xe_mem_type_to_name[]. This is an array of

pointers that should be safe for the event to access. Instead of flagging

this as a bad reference, if a reference points to an array, where the

record field is the index, consider it safe.