ipv4: Turn rt->rt_route_iif into rt->rt_is_input.

That is this value's only use, as a boolean to indicate whether
a route is an input route or not.

So implement it that way, using a u16 gap present in the struct
already.

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

diff --cc include/net/route.h
Simple merge

diff --cc net/ipv4/route.c
Simple merge

diff --cc net/ipv4/xfrm4_policy.c
Simple merge