ipv4: prevent potential spectre v1 gadget in fib_metrics_match()

In the Linux kernel, the following vulnerability has been resolved:

ipv4: prevent potential spectre v1 gadget in fib_metrics_match()

if (!type)
continue;
if (type > RTAX_MAX)
return false;
...
fi_val = fi->fib_metrics->metrics[type - 1];

@type being used as an array index, we need to prevent
cpu speculation or risk leaking kernel memory content.