Fix bug when sorting snapshots

Before this sortsnapshots always tried to sort source snapshots, also
when it was requested to sort target snapshots. This led to errors
when the list of target and source snapshots was not identical.
Fix it by allowing to specify an index when sorting snapshots.

syncoid

@@ -592,7 +592,7 @@ sub syncdataset {
 			my %bookmarks = getbookmarks($sourcehost,$sourcefs,$sourceisroot);
 
 			# check for matching guid of source bookmark and target snapshot (oldest first)
-			foreach my $snap ( sort { sortsnapshots(\%snaps, $b, $a) } keys %{ $snaps{'target'} }) {
+			foreach my $snap ( sort { sortsnapshots(\%snaps, $b, $a, 'target') } keys %{ $snaps{'target'} }) {
 				my $guid = $snaps{'target'}{$snap}{'guid'};
 
 				if (defined $bookmarks{$guid}) {
@@ -682,7 +682,7 @@ sub syncdataset {
 					# if intermediate snapshots are needed we need to find the next oldest snapshot,
 					# do an replication to it and replicate as always from oldest to newest
 					# because bookmark sends doesn't support intermediates directly
-					foreach my $snap ( sort { sortsnapshots(\%snaps, $a, $b) } keys %{ $snaps{'source'} }) {
+					foreach my $snap ( sort { sortsnapshots(\%snaps, $a, $b, 'source') } keys %{ $snaps{'source'} }) {
 						my $comparisonkey = 'creation';
 						if (defined $snaps{'source'}{$snap}{'createtxg'} && defined $bookmark{'createtxg'}) {
 							$comparisonkey = 'createtxg';
@@ -865,7 +865,7 @@ sub syncdataset {
 		# those that exist on the source. Remaining are the snapshots
 		# that are only on the target. Then sort to remove the oldest
 		# snapshots first.
-		my @to_delete = sort { sortsnapshots(\%snaps, $a, $b) } grep {!exists $snaps{'source'}{$_}} keys %{ $snaps{'target'} };
+		my @to_delete = sort { sortsnapshots(\%snaps, $a, $b, 'source') } grep {!exists $snaps{'source'}{$_}} keys %{ $snaps{'target'} };
 		while (@to_delete) {
 			# Create batch of snapshots to remove
 			my $snaps = join ',', splice(@to_delete, 0, 50);
@@ -1486,16 +1486,16 @@ sub readablebytes {
 }
 
 sub sortsnapshots {
-	my ($snaps, $left, $right) = @_;
-	if (defined $snaps->{'source'}{$left}{'createtxg'} && defined $snaps->{'source'}{$right}{'createtxg'}) {
-		return $snaps->{'source'}{$left}{'createtxg'} <=> $snaps->{'source'}{$right}{'createtxg'};
+	my ($snaps, $left, $right, $idx) = @_;
+	if (defined $snaps->{$idx}{$left}{'createtxg'} && defined $snaps->{$idx}{$right}{'createtxg'}) {
+		return $snaps->{$idx}{$left}{'createtxg'} <=> $snaps->{$idx}{$right}{'createtxg'};
 	}
-	return $snaps->{'source'}{$left}{'creation'} <=> $snaps->{'source'}{$right}{'creation'};
+	return $snaps->{$idx}{$left}{'creation'} <=> $snaps->{$idx}{$right}{'creation'};
 }
 
 sub getoldestsnapshot {
 	my $snaps = shift;
-	foreach my $snap (sort { sortsnapshots($snaps, $a, $b) } keys %{ $snaps{'source'} }) {
+	foreach my $snap (sort { sortsnapshots($snaps, $a, $b, 'source') } keys %{ $snaps{'source'} }) {
 		# return on first snap found - it's the oldest
 		return $snap;
 	}
@@ -1509,7 +1509,7 @@ sub getoldestsnapshot {
 
 sub getnewestsnapshot {
 	my $snaps = shift;
-	foreach my $snap (sort { sortsnapshots($snaps, $b, $a) } keys %{ $snaps{'source'} }) {
+	foreach my $snap (sort { sortsnapshots($snaps, $b, $a, 'source') } keys %{ $snaps{'source'} }) {
 		# return on first snap found - it's the newest
 		writelog('INFO', "NEWEST SNAPSHOT: $snap");
 		return $snap;
@@ -1688,7 +1688,7 @@ sub pruneoldsyncsnaps {
 
 sub getmatchingsnapshot {
 	my ($sourcefs, $targetfs, $snaps) = @_;
-	foreach my $snap ( sort { sortsnapshots($snaps, $b, $a) } keys %{ $snaps{'source'} }) {
+	foreach my $snap ( sort { sortsnapshots($snaps, $b, $a, 'source') } keys %{ $snaps{'source'} }) {
 		if (defined $snaps{'target'}{$snap}) {
 			if ($snaps{'source'}{$snap}{'guid'} == $snaps{'target'}{$snap}{'guid'}) {
 				return $snap;