debugtrace: fix buffer allocation

Commit 6338c9ead9 ("debugtrace: add per-cpu buffer option") had
a rebase error when using per-cpu buffers: the global buffer address
would always be set to the one of the last per-cpu buffer allocated.

The result would be that when dumping the buffers the last cpu's buffer
is always shown as empty as those entries are printed in the global
buffer's dump already.

Fix that.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/common/debugtrace.c

@@ -259,7 +259,6 @@ static void debugtrace_alloc_buffer(struct debugtrace_data **ptr,
 
     memset(data, '\0', debugtrace_bytes + sizeof(*data));
 
-    dt_data = data;
     *ptr = data;
 }