f2fs: support superblock checksum

Now we support crc32 checksum for superblock.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Junling Zheng <zhengjunling@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/include/linux/f2fs_fs.h b/include/linux/f2fs_fs.h
index 1d4b1962..1db13ff 100644
--- a/include/linux/f2fs_fs.h
+++ b/include/linux/f2fs_fs.h
@@ -109,7 +109,8 @@ struct f2fs_super_block {
 	struct f2fs_device devs[MAX_DEVICES];	/* device list */
 	__le32 qf_ino[F2FS_MAX_QUOTAS];	/* quota inode numbers */
 	__u8 hot_ext_count;		/* # of hot file extension */
-	__u8 reserved[314];		/* valid reserved region */
+	__u8 reserved[310];		/* valid reserved region */
+	__le32 crc;			/* checksum of superblock */
 } __packed;
 
 /*