f2fs: fix memory alignment to support 32bit

In 32bit system, 64-bits key breaks memory alignment.
This fixes the commit "f2fs: support 64-bits key in f2fs rb-tree node entry".

Reported-by: Nicolas Chauvet <kwizart@gmail.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>