f2fs: fix wrong written_valid_blocks counting

commit c79b7ff1d3c7710c23d8828a69d8cbc5597ad19f upstream.

Previously, written_valid_blocks was got by ckpt->valid_block_count. But if
the last checkpoint has some NEW_ADDR due to power-cut, we can get wrong value.
Fix it to get the number from actual written block count from sit entries.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>