net: ipv6: Rework net_ipv6_nbr_add

This makes net_ipv6_nbr_add able to update entries so it can be reused
when receiving RA or NS requests, so it now performs a lookup before
creating a new entry (using nbr_new to reuse more code) and in case it
finds a match attempts to update the lladdr.

Change-Id: I305a67a955e037cbbb862fef947a5fcfe131507c
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>