journal: Properly track the number of allocated windows.

Checks were already in place to make sure that the number of
windows was limited to 64, but the count was never incremented
or decremented.

src/journal/mmap-cache.c

@@ -130,6 +130,7 @@ static void window_free(Window *w) {
         assert(w);
 
         window_unlink(w);
+        w->cache->n_windows--;
         free(w);
 }
 
@@ -157,6 +158,7 @@ static Window *window_add(MMapCache *m) {
                 w = new0(Window, 1);
                 if (!w)
                         return NULL;
+                m->n_windows++;
         } else {
 
                 /* Reuse an existing one */