net/ipv6: Renaming IPv6 neighbor state related macros and function

Refining the names around IPv6's neighbor states to differentiate them
easily from any net_nbr related names (which are not tighten to IPv6).

Change-Id: Ibc24df2a9485477a53fe5fe1c8f993f0fcd91635
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>