ipv4: Rearrange how ip_route_newports() gets port keys.

ip_route_newports() is the only place in the entire kernel that
cares about the port members in the routing cache entry's lookup
flow key.

Therefore the only reason we store an entire flow inside of the
struct rtentry is for this one special case.

Rewrite ip_route_newports() such that:

1) The caller passes in the original port values, so we don't need
   to use the rth->fl.fl_ip_{s,d}port values to remember them.

2) The lookup flow is constructed by hand instead of being copied
   from the routing cache entry's flow.

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