powerpc/pseries: papr-phy-attest - validate cmd.length, plug mem leak

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

powerpc/pseries: papr-phy-attest - validate cmd.length, plug mem leak

In papr_phy_attest_create_handle(), the params->cmd.length is not validated before use, which can result in a buffer overlow. Check it and return -EINVAL if it is either 0 or exceeds sizeof(params->cmd).

Also, params is freed on the success path but not error. Free it on errors after memory allocation. And free it on negative fd.