net: fix potential neighbour race in dst_ifdown()

Followup of commit f2c31e32 (fix NULL dereferences in
check_peer_redir()).

We need to make sure dst neighbour doesnt change in dst_ifdown().

Fix some sparse errors.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>