Network stack cleanup: IPv6

[tbursztyka]

ipv6.c needs cleanup:
. nbr logic should be reworked using sys_slist instead of an array. (and why not putting the code in ipv6_nd.c)
. revise struct net_ipv6_nbr_data: k_delayed_work is an heavy object (40 bytes). Check what has been done in subsys/net/l2/ethernet/arp.c: their, only 1 k_work is being used to handle timeout. Same for pending/addr attributes, check arp_entry recent changes.

icmvp6.c as well:
. use net_ipv6_create/finalize instead of redoing the ipv6 headers there (get rid of setup_ipv6_header)
. use net_frag_linearize() in net_icmpv6_get_<...>_hdr()

declutter using #8723 rules

[rveerama1]

Most of the issues are addressed already. So closing it.