flow: Move __get_hash_from_flowi{4,6} into flow_dissector.c

These cannot live in net/core/flow.c which only builds when XFRM is
enabled.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/core/flow.c
Simple merge

diff --cc net/core/flow_dissector.c
Simple merge