xfrm: rework default policy structure

[ Upstream commit b58b1f56 ]

This is a follow up of commit f8d858e6 ("xfrm: make user policy API
complete"). The goal is to align userland API to the internal structures.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>