flow_dissector: Fix function argument ordering dependency

Commit c6cc1ca7 ("flowi: Abstract out functions to get flow hash
based on flowi") introduced a bug in __skb_set_sw_hash where we
require a dependency on evaluating arguments in a function in order.
There is no such ordering enforced in C, so this incorrect. This
patch fixes that by splitting out the arguments. This bug was
found via a compiler warning that keys may be uninitialized.

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