xen/trace: Don't dump offline CPUs in debugtrace_dump_worker()

The 'T' debugkey reliably wedges on one of my systems, which has a sparse
APIC_ID layout due to a non power-of-2 number of cores per socket.  The
per_cpu(dt_cpu_data, cpu) calcution falls over the deliberately non-canonical
poison value.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: abf378e648
master date: 2020-05-22 20:04:46 +0100

xen/common/debugtrace.c

@@ -94,7 +94,7 @@ static void debugtrace_dump_worker(void)
 
     debugtrace_dump_buffer(dt_data, "global");
 
-    for ( cpu = 0; cpu < nr_cpu_ids; cpu++ )
+    for_each_online_cpu ( cpu )
     {
         char buf[16];