vdpa: Add features attr to vdpa_nl_policy for nlattr length check

In the Linux kernel, the following vulnerability has been resolved:

vdpa: Add features attr to vdpa_nl_policy for nlattr length check

The vdpa_nl_policy structure is used to validate the nlattr when parsing

the incoming nlmsg. It will ensure the attribute being described produces

a valid nlattr pointer in info->attrs before entering into each handler

in vdpa_nl_ops.

That is to say, the missing part in vdpa_nl_policy may lead to illegal

nlattr after parsing, which could lead to OOB read just like CVE-2023-3773.

This patch adds the missing nla_policy for vdpa features attr to avoid

such bugs.