md-cluster/bitmap: fix wrong page num in bitmap_file_clear_bit and bitmap_file_set_bit
The pnum passed to set_page_attr and test_page_attr should from
0 to storage.file_pages - 1, but bitmap_file_set_bit and
bitmap_file_clear_bit call set_page_attr and test_page_attr with
page->index parameter while page->index has already added node_offset
before.
So we need to minus node_offset in both bitmap_file_clear_bit
and bitmap_file_set_bit.