fellow_log: logwatcher flush is also needed when dsk needs to be freed

the periodic flush via FLW_MAYFLUSH (which writes a new header) also needs to be
active when there are disk blocks to be freed, otherwise we might deadlock.

src/fellow_log.c

@@ -6145,6 +6145,10 @@ fellow_logwatcher_need(struct fellow_fd *ffd, struct flw_need_why *why)
 			return (need);
 	}
 
+	if (lbuf->regions_to_free != NULL &&
+	    regionlist_used(lbuf->regions_to_free) > 0)
+		return (FLW_MAYFLUSH);
+
 	if (lbuf->space == 0 || lbuf->n == 0)
 		return (FLW_NEED_NONE);
 
@@ -6168,7 +6172,11 @@ fellow_logwatcher_flush(struct fellow_fd *ffd, unsigned can)
 {
 
 	DBG("concurrent n=%u state=%u", ffd->logbuf->n, ffd->logbuf->state);
-	if (ffd->logbuf->n == 0 || ffd->logbuf->state != LBUF_OPEN)
+	if (ffd->logbuf->state != LBUF_OPEN)
+		return;
+	if ((ffd->logbuf->regions_to_free == NULL ||
+	    regionlist_used(ffd->logbuf->regions_to_free) == 0) &&
+	    ffd->logbuf->n == 0)
 		return;
 
 	// logmtx is held