net: ipv6: Print IPv6 neighbor state changes

In order to be able to debug IPv6 neighbor state changes,
pretty print the state names when setting the new state.

Change-Id: I64e0b4173eaa63c9715c17420a008602759aa4fa
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>