refperf: Dynamically allocate thread-summary output buffer

Currently, the buffer used to accumulate the thread-summary output is
fixed size, which will cause problems if someone decides to run on a large
number of PCUs.  This commit therefore dynamically allocates this buffer.

[ paulmck: Fix memory allocation as suggested by KASAN. ]
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/refperf.c

@@ -301,9 +301,12 @@ u64 process_durations(int n)
 	int i;
 	struct reader_task *rt;
 	char buf1[64];
-	char buf[512];
+	char *buf;
 	u64 sum = 0;
 
+	buf = kmalloc(128 + nreaders * 32, GFP_KERNEL);
+	if (!buf)
+		return 0;
 	buf[0] = 0;
 	sprintf(buf, "Experiment #%d (Format: <THREAD-NUM>:<Total loop time in ns>)",
 		exp_idx);
@@ -322,6 +325,7 @@ u64 process_durations(int n)
 
 	PERFOUT("%s\n", buf);
 
+	kfree(buf);
 	return sum;
 }