net: flow_dissector: small optimizations in IPv4 dissect

By moving code around, we avoid :

1) A reload of iph->ihl (bit field, so needs a mask)

2) A conditional test (replaced by a conditional mov on x86)
   Fast path loads iph->protocol anyway.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>