Attempt forced trim when stopping an overrun flusher

Fixes a rare hang happening in the event of journal space running out without
new work to do for flushers except the current sector.
The hang could be reproduced more or less consistently with very slow drives.

blockstore_flush.cpp

@@ -223,6 +223,7 @@ bool journal_flusher_co::loop()
 resume_0:
     if (!flusher->flush_queue.size() || !flusher->dequeuing)
     {
+stop_flusher:
         if (flusher->trim_wanted > 0 && flusher->journal_trim_counter > 0)
         {
             // Attempt forced trim
@@ -327,9 +328,7 @@ resume_0:
 #ifdef BLOCKSTORE_DEBUG
                     printf("No older flushes, stopping\n");
 #endif
-                    flusher->dequeuing = false;
-                    wait_state = 0;
-                    return true;
+                    goto stop_flusher;
                 }
             }
         }