Syncoid: bookmarks not created when latest snapshot is used

[pavhofman]

Scenario: source gets regularly updated and each update is marked with a new snapshot. The previous snapshot on the source is destroyed afterwards.

syncoid creates the first bookmark on the source, but no more bookmarks afterwards. As a result zfs keeps sending increasingly more data each run as the target gets rolled-back to the target snapshot corresponding to the source bookmark which gets progressively older. IMO it should use a bookmark corresponding to the latest synchronized snapshot instead.

I tracked the problem to this check

sanoid/syncoid

Lines 769 to 772 in ef601eb

	if ($matchingsnap eq $newsyncsnap) {
	# edge case: bookmark replication used the latest snapshot
	return 0;
	}

which returns from the syncdataset method before the code creating the bookmarks

sanoid/syncoid

Line 814 in ef601eb

if (defined $args{'no-sync-snap'}) {

gets called.