ipv4: If fib metrics are default, no need to grab ref to FIB info.

ipv4: If fib metrics are default, no need to grab ref to FIB info.

The fib metric memory in this case is static in the kernel image,
so we don't need to reference count it since it's never going
to go away on us.

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