af_key: return error if pfkey_xfrm_policy2msg_prep() fails

The original code saved the error value but just returned 0 in the end.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Jamal Hadi Salim <hadi@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>