f2fs: fix wrong i_atime recovery

Shouldn't update in-memory i_atime with on-disk i_mtime of inode when
recovering inode.

Shuoran found this bug which is hidden for a long time, honour is belong
to him.

Signed-off-by: Shuoran Liu <liushuoran@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>