ipv4: Fix peer validation on cached lookup.

If ipv4_valdiate_peer() fails during a cached entry lookup,
we'll NULL derer since the loop iterator assumes rth is not
NULL.

Letting this be handled as a failure is just bogus, so just make it
not fail.  If we have trouble getting a non-NULL neighbour for the
redirected gateway, just restore the original gateway and continue.

The very next use of this cached route will try again.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>