tracing: ring-buffer: Fix to check event length before using

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

tracing: ring-buffer: Fix to check event length before using

Check the event length before adding it for accessing next index in

rb_read_data_buffer(). Since this function is used for validating

possibly broken ring buffers, the length of the event could be broken.

In that case, the new event (e + len) can point a wrong address.

To avoid invalid memory access at boot, check whether the length of

each event is in the possible range before using it.