net: ipv6: Wrong timer was used when reachability timer expired

The send_ns timer was used instead of reachable timer when
the reachability timer was expired. This meant that the
reachability timer was never restarted after it expired once.

Change-Id: Iccbd6d019d1106adca60f7319bc4c547cdb1d924
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>