net: ipv6: Send Neighbor Solicitations in PROBE state as unicast

According to RFC 4861, ch. 7.3.3:

 "Upon entering the PROBE state, a node sends a unicast Neighbor
  Solicitation message to the neighbor using the cached link-layer
  address."

Zephyr's implementation was not compliant with behavior, as instead of
sending a unicast probe for reachability confirmation, it was sending a
multicast packet instead. This commit fixes it.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>