net: ipv6: Properly enter DELAY state in NUD

We did not handle NUD (Neighbor Unreachability Detection)
properly and did not go to DELAY state when neighbor
reachability was checked.

Change-Id: I127d25124aa806c44ff37c1355cc3db7fb25b2d2
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>