f2fs: support large nat bitmap

Previously, we will store all nat version bitmap in checkpoint pack block,
so our total node entry number has a limitation which caused total node
number can not exceed (3900 * 8) block * 455 node/block = 14196000. So
that once user wants to create more nodes in large size image, it becomes
a bottleneck, that's unreasonable.

This patch detects the new layout of nat/sit version bitmap in image in
order to enable supporting large nat bitmap.

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