xfrm: use xfrm direction when lookup policy

because xfrm policy direction has same value with corresponding
flow direction, so this problem is covered.

In xfrm_lookup and __xfrm_policy_check, flow_cache_lookup is used to
accelerate the lookup.

Flow direction is given to flow_cache_lookup by policy_to_flow_dir.

When the flow cache is mismatched, callback 'resolver' is called.

'resolver' requires xfrm direction,
so convert direction back to xfrm direction.

Signed-off-by: Baker Zhang <baker.zhang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>