ipv4: Cleanup struct net dereference in rt_intern_hash

There's no need in getting it 3 times and gcc isn't smart enough
to understand this himself.

This is just a cleanup before the fix (next patch).

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>