f2fs: change le32 to le16 of f2fs_inode->i_extra_size

In the structure of f2fs_inode, i_extra_size's type is __le16,
so we should keep type consistent when using it.

Fixes: 704956ecf5bc ("f2fs: support inode checksum")
Signed-off-by: Zhikang Zhang <zhangzhikang1@huawei.com>
Signed-off-by: Yunlei He <heyunlei@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>