net: Checks skb_dst to be NULL in inet_iif

In inet_iif check if skb_rtable is NULL for the skb and return
skb->skb_iif if it is.

This change allows inet_iif to be called before the dst
information has been set in the skb (e.g. when doing socket based
UDP GRO).

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>