get_compactions().size() may return 0 while there are
non-zero tasks to stop.
Some tasks may not be marked as `compaction_running` since
they are either:
- postponed (due to compaction manger throttling of regular compaction)
- sleeping before retry.
In both cases we still want to stop them so the log message
should reflect both the number of ongoing compactions
and the actual number of tasks we're stopping.
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>