exfat: fix inode->i_blocks for non-512 byte sector size device

inode->i_blocks is not real number of blocks, but 512 byte ones.

Fixes: 98d917047e8b ("exfat: add file operations")
Cc: stable@vger.kernel.org # v5.7+
Reported-by: Wang Yugui <wangyugui@e16-tech.com>
Tested-by: Wang Yugui <wangyugui@e16-tech.com>
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Reviewed-by: Andy Wu <Andy.Wu@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>