LU-14731 mdd: clear orphans changelog entries

LU-14731 mdd: clear orphans changelog entries

In mdd_changelog_llog_init(), adjust the orphan changelog index logic
to account for the case when no users are registered. Add sanity
test_160n() to verify this.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I03b0c1002a0e16f26af8ec23bf06c9a07dec858a
Reviewed-on: https://review.whamcloud.com/43901
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>