af_key: fix SADB_X_SPDDELETE response

When deleting an SPD entry using SADB_X_SPDDELETE, c.data.byid is not
initialized to zero in pfkey_spddelete(). Thus, key_notify_policy()
responds with a PF_KEY message of type SADB_X_SPDDELETE2 instead of
SADB_X_SPDDELETE.

Signed-off-by: Tobias Brunner <tobias.brunner@strongswan.org>
Signed-off-by: David S. Miller <davem@davemloft.net>