net: xfrm: Amend XFRMA_SEC_CTX nla_policy structure

According to all consumers code of attrs[XFRMA_SEC_CTX], like

* verify_sec_ctx_len(), convert to xfrm_user_sec_ctx*
* xfrm_state_construct(), call security_xfrm_state_alloc whose prototype
is int security_xfrm_state_alloc(.., struct xfrm_user_sec_ctx *sec_ctx);
* copy_from_user_sec_ctx(), convert to xfrm_user_sec_ctx *
...

It seems that the expected parsing result for XFRMA_SEC_CTX should be
structure xfrm_user_sec_ctx, and the current xfrm_sec_ctx is confusing
and misleading (Luckily, they happen to have same size 8 bytes).

This commit amend the policy structure to xfrm_user_sec_ctx to avoid
ambiguity.

Fixes: cf5cb79f ("[XFRM] netlink: Establish an attribute policy")
Signed-off-by: Lin Ma <linma@zju.edu.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>