f2fs: mark recovery flag correctly in read_raw_super_block()

On the combination of first fail and second success,
we will miss to mark recovery flag because currently
we reuse err variable in the loop.

Signed-off-by: Chengguang Xu <cgxu519@zoho.com.cn>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>