lsa: update _closed_occupancy after freeing all segments

_closed_occupancy will be used when a region is removed from its region
group, make sure that it is accurate.

Signed-off-by: Paweł Dziepak <pdziepak@scylladb.com>
(cherry picked from commit 338fd34770)

utils/logalloc.cc

@@ -1211,6 +1211,7 @@ public:
             assert(seg->is_empty());
             free_segment(seg);
         }
+        _closed_occupancy = {};
         if (_active) {
             assert(_active->is_empty());
             free_segment(_active);