metrics: Fix percentage difference row

Fix the percentage difference calculation for the metrics report generation.

Fixes kata-containers#709

Signed-off-by: Gabriela Cervantes <gabriela.cervantes.tellez@intel.com>

metrics/report/report_dockerfile/lifecycle-time.R

@@ -98,7 +98,8 @@ if (length(resultdirs) == 2) {
 	# This is a touch hard wired - but we *know* we only have two
 	# datasets...
 	for( i in 1:5) {
-		val = ((as.double(rstats[1,i]) / as.double(rstats[2,i])) * 100) - 100
+		difference = as.double(rstats[2,i]) - as.double(rstats[1,i])
+		val = 100 * (difference/as.double(rstats[1,i]))
 		diff[i] = paste(round(val, digits=2), "%", sep=" ")
 	}
 

metrics/report/report_dockerfile/memory-footprint.R

@@ -82,9 +82,11 @@ if (length(resultdirs) == 2) {
 	# This is a touch hard wired - but we *know* we only have two
 	# datasets...
 	diff=c("diff")
-	val = ((as.double(rstats[1,2]) / as.double(rstats[2,2])) * 100) - 100
+	difference = (as.double(rstats[2,2]) - as.double(rstats[1,2]))
+	val = 100 * (difference/as.double(rstats[1,2]))
 	diff[2] = round(val, digits=2)
-	val = ((as.double(rstats[1,3]) / as.double(rstats[2,3])) * 100) - 100
+	difference = (as.double(rstats[2,3]) - as.double(rstats[1,3]))
+	val = 100 * (difference/as.double(rstats[1,3]))
 	diff[3] = round(val, digits=2)
 	rstats=rbind(rstats, diff)