libdm: report: fix invalid JSON if using DM_REPORT_OUTPUT_MULTIPLE_TIMES and selection

libdm: report: fix invalid JSON if using DM_REPORT_OUTPUT_MULTIPLE_TIMES and selection

When reporting in JSON format, we need to be able to find the 'last
displayed row', not just 'last row' as we did before. This is used
to decide whether to put the JSON_SEPARATOR (the ',' character)
between the lines when reporting in JSON format.

This is mainly important in case we use a combination of JSON format
and a report marked with DM_REPORT_OUTPUT_MULTIPLE_TIMES flag.
Such report may be reused several times with different selection
criteria each time. In that case, the report always contains all lines
in memory, even though some of them do not need to pass the selection
criteria that are currently used.

Without DM_REPORT_OUTPUT_MULTIPLE_TIMES flag, the report only contains
the lines that have passed the selection criteria, so the this wasn't
an issue in this case.

Fix suggested by Lars Ellenberg and reported here: https://github.com/lvmteam/lvm2/issues/130