Fix journal dirty_start position tracking and some debug prints

Fixes two bugs found during HDD testing :-)
1) OSD crashed with "BUG: Attempt to overwrite used offset of the journal" during
   `fio -bs=900k -iodepth=128` test with 16 MB journal
2) OSD stalled during `fio -bs=512k -iodepth=128` test with 64 MB journal

tests/run_3osds.sh

@@ -82,7 +82,9 @@ wait_up()
     done
 }
 
-wait_up 60
+if [[ $OSD_COUNT -gt 0 ]]; then
+    wait_up 60
+fi
 
 try_reweight()
 {