f2fs: fix to update new block address correctly for OPU

Previously, we allocated a new block address for OPU mode in direct_IO.

But the new address couldn't be assigned to @map->m_pblk correctly.

This patch fix it.

Cc: <stable@vger.kernel.org>
Fixes: 511f52d02f05 ("f2fs: allow out-place-update for direct IO in LFS mode")
Signed-off-by: Jia Zhu <zhujia13@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>