torture: Grace periods do not piggyback off of themselves

The rcuperf trace-event processing counted every "done" trace event
as a piggyback, which is incorrect because the task that started the
grace period didn't piggyback at all.  This commit fixes this problem
by recording the task that started a given grace period and ignoring
that task's "done" record for that grace period.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

tools/testing/selftests/rcutorture/bin/kvm-recheck-rcuperf-ftrace.sh

@@ -44,6 +44,7 @@ $8 == "start" {
 	starttask = $1;
 	starttime = $3;
 	startseq = $7;
+	seqtask[startseq] = starttask;
 }
 
 $8 == "end" {
@@ -62,7 +63,7 @@ $8 == "end" {
 	}
 }
 
-$8 == "done" {
+$8 == "done" && seqtask[$7] != $1 {
 	piggybackcnt[$1]++;
 }