Make the end variable be really at the end of bss, and delete + 4096 in freerange()

kernel/kalloc.c

@@ -35,7 +35,6 @@ freerange(void *pa_start, void *pa_end)
 {
   char *p;
   p = (char*)PGROUNDUP((uint64)pa_start);
-  p += 4096; // XXX I can't get kernel.ld to place end beyond the last bss symbol.
   for(; p + PGSIZE <= (char*)pa_end; p += PGSIZE)
     kfree(p);
 }

kernel/kernel.ld

@@ -26,7 +26,7 @@ SECTIONS
   }
   .bss : {
     *(.bss)
+    *(.sbss*)
      PROVIDE(end = .);
   }
-
 }