ipv6 netevent: Remove old_neigh from netevent_redirect.

The only user is cxgb3 driver.

old_neigh is used to check device change, but it must not happen
on redirect.  In this sense, we can remove old_neigh argument.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>