heal: Fix passing healing opts (#9756)

Manual healing (as background healing) creates a heal task with a
possiblity to override healing options, such as deep or normal mode.

Use a pointer type in heal opts so nil would mean use the default
healing options.

cmd/global-heal.go

@@ -128,7 +128,7 @@ func deepHealObject(objectPath string) {
 
 	bgSeq.sourceCh <- healSource{
 		path: objectPath,
-		opts: madmin.HealOpts{ScanMode: madmin.HealDeepScan},
+		opts: &madmin.HealOpts{ScanMode: madmin.HealDeepScan},
 	}
 }