ring_buffer: map to cpu not page

My original patch had a compile bug when NUMA was configured. I
referenced cpu when it should have been cpu_buffer->cpu.

Ingo quickly fixed this bug by replacing cpu with 'i' because that
was the loop counter. Unfortunately, the 'i' was the counter of
pages, not CPUs. This caused a crash when the number of pages allocated
for the buffers exceeded the number of pages, which would usually
be the case.
Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/ring_buffer.c

@@ -232,7 +232,7 @@ static int rb_allocate_pages(struct ring_buffer_per_cpu *cpu_buffer,
 
 	for (i = 0; i < nr_pages; i++) {
 		page = kzalloc_node(ALIGN(sizeof(*page), cache_line_size()),
-				    GFP_KERNEL, cpu_to_node(i));
+				    GFP_KERNEL, cpu_to_node(cpu_buffer->cpu));
 		if (!page)
 			goto free_pages;
 		list_add(&page->list, &pages);