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/memory-footprint.R

@@ -82,9 +82,13 @@ 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
+	diference = abs(as.double(rstats[1,2]) - as.double(rstats[2,2]))
+	average = (as.double(rstats[1,2]) + as.double(rstats[2,2])) / 2
+	val = 100 * (diference/average)
 	diff[2] = round(val, digits=2)
-	val = ((as.double(rstats[1,3]) / as.double(rstats[2,3])) * 100) - 100
+	diference = abs(as.double(rstats[1,3]) - as.double(rstats[2,3]))
+	average = (as.double(rstats[1,3]) + as.double(rstats[2,3])) / 2
+	val = 100 * (diference/average)
 	diff[3] = round(val, digits=2)
 	rstats=rbind(rstats, diff)