xfrm_policy: kill flow to policy dir conversion

XFRM_POLICY_IN/OUT/FWD are identical to FLOW_DIR_*, so gcc already
removed this function as its just returns the argument.  Again, no
code change.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>