f2fs: skip searching non-exist range in truncate_hole

Let's skip entire non-exist area to speed up truncate_hole by
using get_next_page_offset.

Signed-off-by: Weichao Guo <guoweichao@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>