ipv6: Fix OOPS in ip6_dst_lookup_tail().

This fixes kernel bugzilla 11469: "TUN with 1024 neighbours:
ip6_dst_lookup_tail NULL crash"

dst->neighbour is not necessarily hooked up at this point
in the processing path, so blindly dereferencing it is
the wrong thing to do.  This NULL check exists in other
similar paths and this case was just an oversight.

Also fix the completely wrong and confusing indentation
here while we're at it.

Based upon a patch by Evgeniy Polyakov.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>