net: rps: skip fragment when computing rxhash

Fragmented IP packets may have no transfer header, so when computing
rxhash, we should skip them.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>