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.

Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>