ipv4: Use less conflicting local var name in change_nexthops() loop macro.

As noticed by H Hartley Sweeten, since change_nexthops() uses 'nh'
as it's iterator variable, it can conflict with other existing
local vars.

Use "nexthop_nh" to avoid the conflict and make it easier to figure
out where this magic variable comes from.

Signed-off-by: David S. Miller <davem@davemloft.net>