x86: UV BAU: Avoid NULL pointer reference in ptc_seq_show

In init_per_cpu(), when get_cpu_topology() fails, init_per_cpu_tunables()
is not called afterwards. This means that bau_control->statp is NULL.
If a user then reads /proc/sgi_uv/ptc_statistics ptc_seq_show() references
a NULL pointer. Therefore, since uv_bau_init calls set_bau_off when
init_per_cpu() fails, we add code that detects when the bau is off in
ptc_seq_show() to avoid referencing a NULL pointer.

Signed-off-by: James Custer <jcuster@sgi.com>
Cc: Russ Anderson <rja@sgi.com>
Link: http://lkml.kernel.org/r/1414952199-185319-2-git-send-email-jcuster@sgi.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/platform/uv/tlb_uv.c

@@ -1384,6 +1384,10 @@ static int ptc_seq_show(struct seq_file *file, void *data)
 	}
 	if (cpu < num_possible_cpus() && cpu_online(cpu)) {
 		bcp = &per_cpu(bau_control, cpu);
+		if (bcp->nobau) {
+			seq_printf(file, "cpu %d bau disabled\n", cpu);
+			return 0;
+		}
 		stat = bcp->statp;
 		/* source side statistics */
 		seq_printf(file,