heal: Check if all parts are available and valid (#3967)

In the algorithm to check if an object requires healing, in addition to
checking if all disks have xl.json present we should check if all parts
of the object are present and have valid blake2b checksums.

Also fixed a minor compilation error in heal-objects-list.go.

cmd/xl-v1-healing.go

@@ -322,7 +322,7 @@ func healObject(storageDisks []StorageAPI, bucket string, object string, quorum
 		return toObjectErr(reducedErr, bucket, object)
 	}
 
-	if !xlShouldHeal(partsMetadata, errs) {
+	if !xlShouldHeal(storageDisks, partsMetadata, errs, bucket, object) {
 		// There is nothing to heal.
 		return nil
 	}