nilfs2: use huge_encode_dev/huge_decode_dev

This replaces uses of new_encode_dev/new_decode_dev with their 64-bit
counterparts, huge_encode_dev/huge_decode_dev respectively.

This is just for clarification and has no impact on the disk format.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>