tracing: Initialize scratch_size to zero to prevent UB

In allocate_trace_buffer() the following code:

  buf->buffer = ring_buffer_alloc_range(size, rb_flags, 0,
				      tr->range_addr_start,
				      tr->range_addr_size,
				      struct_size(tscratch, entries, 128));

  tscratch = ring_buffer_meta_scratch(buf->buffer, &scratch_size);
  setup_trace_scratch(tr, tscratch, scratch_size);

Has undefined behavior if ring_buffer_alloc_range() fails because
"scratch_size" is not initialize. If the allocation fails, then
buf->buffer will be NULL. The ring_buffer_meta_scratch() will return
NULL immediately if it is passed a NULL buffer and it will not update
scratch_size. Then setup_trace_scratch() will return immediately if
tscratch is NULL.

Although there's no real issue here, but it is considered undefined
behavior to pass an uninitialized variable to a function as input, and
UBSan may complain about it.

Just initialize scratch_size to zero to make the code defined behavior and
a little more robust.

Link: https://lore.kernel.org/all/[email protected]/

Reported-by: Dan Carpenter <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

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