[IPV4,IPV6]: replace handmade list with hlist in IPv{4,6} reassembly

Both of ipq and frag_queue have *next and **prev, and they can be replaced
with hlist. Thanks Arnaldo Carvalho de Melo for the suggestion.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Acked-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>