ipv4: fix fib metrics

Alessandro Suardi reported that we could not change route metrics :

ip ro change default .... advmss 1400

This regression came with commit 9c150e82ac50 (Allocate fib metrics
dynamically). fib_metrics is no longer an array, but a pointer to an
array.

Reported-by: Alessandro Suardi <alessandro.suardi@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Alessandro Suardi <alessandro.suardi@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>