sys_msync

2. clean codes in filemap.c

1. readahead pages which has in pagecache.
2. delete repeated put_page in free_mapping.

1. free
2. hwclock

1. When only page which has index 0 in pagecache, put it's address in the root->rnode.
2. add codes:
  if(root->height == 0 && root->rnode){
    put_page((page_t *)root->rnode);
  }
If we donot do this, the bug is occurs when the i_mapping is free, but the page is not put, so it still in
page reclaiming list, when the page is reclaimed, we need to remove it frome pagecache, but the i_mapping is
freed.