net: ipv6: Send NS when neighbor is added

In order to verify that the neighbor is reachable, send
neighbor solicitation message to neighbor which should then
send neighbor advertisement to us so that we can then mark
it reachable.

Change-Id: I2c45ee4a6982c698ef6c8fa1acccb995aadf8ee6
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>