f2fs: check cur_valid_map_mir & raw_sit block count when flush sit entries

We should check valid_map_mir and block count to ensure
the flushed raw_sit is correct.

Signed-off-by: Zhikang Zhang <zhangzhikang1@huawei.com>
Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>