Fix another rare journal flush stall

src/blockstore_flush.cpp

@@ -162,7 +162,8 @@ void journal_flusher_t::mark_trim_possible()
     if (trim_wanted > 0)
     {
         dequeuing = true;
-        journal_trim_counter++;
+        if (!journal_trim_counter)
+            journal_trim_counter = journal_trim_interval;
         bs->ringloop->wakeup();
     }
 }

src/blockstore_read.cpp

@@ -286,7 +286,10 @@ void blockstore_impl_t::handle_read_event(ring_data_t *data, blockstore_op_t *op
                 {
                     auto used = --journal.used_sectors[rv.journal_sector-1];
                     if (used == 0)
+                    {
                         journal.used_sectors.erase(rv.journal_sector-1);
+                        flusher->mark_trim_possible();
+                    }
                 }
             }
         }

src/blockstore_rollback.cpp

@@ -127,7 +127,6 @@ resume_4:
     {
         mark_rolled_back(*v);
     }
-    flusher->mark_trim_possible();
     // Acknowledge op
     op->retval = 0;
     FINISH_OP(op);
@@ -232,6 +231,7 @@ void blockstore_impl_t::erase_dirty(blockstore_dirty_db_t::iterator dirty_start,
         if (used == 0)
         {
             journal.used_sectors.erase(dirty_it->second.journal_sector);
+            flusher->mark_trim_possible();
         }
         if (dsk.clean_entry_bitmap_size > sizeof(void*))
         {