packet lib: rename icmp6 to icmpv6

- renamed variables "ICMP6_XXX" to "ICMPV6_XXX"

Signed-off-by: HIYAMA Manabu <hiyama.manabu@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>