ipv4: Fix bugs in fib_compute_spec_dst().

Based upon feedback from Julian Anastasov.

1) Use route flags to determine multicast/broadcast, not the
   packet flags.

2) Leave saddr unspecified in flow key.

3) Adjust how we invoke inet_select_addr().  Pass ip_hdr(skb)->saddr as
   second arg, and if it was zeronet use link scope.

4) Use loopback as input interface in flow key.

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