af_key: initialize xfrm encap_oa

Currently encap_oa is left uninitialized, so it contains garbage data which
is visible to userland via Netlink. Initialize it by zeroing it out.

Signed-off-by: Timo Teras <timo.teras@iki.fi>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>