[IPV6]: Make ndisc_flow_init() common for later use.

For later use, this patch is renaming ndisc_flow_init() to
icmpv6_flow_init() and putting it in common place.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

diff --cc include/linux/icmpv6.h
Simple merge

diff --cc net/ipv6/icmp.c
Simple merge

diff --cc net/ipv6/ndisc.c
Simple merge