net/flow_dissector: Make dissector_uses_key() and skb_flow_dissector_target() public

Will be used in a following patch to query if a key is being used, and
what it's value in the target object.

Acked-by: John Fastabend <john.r.fastabend@intel.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Amir Vadai <amir@vadai.me>
Signed-off-by: David S. Miller <davem@davemloft.net>