KVM: SVM: Fix an error code in sev_gmem_post_populate()

The copy_from_user() function returns the number of bytes which it
was not able to copy.  Return -EFAULT instead.

Fixes: dee5a47c ("KVM: SEV: Add KVM_SEV_SNP_LAUNCH_UPDATE command")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Message-ID: <20240612115040.2423290-4-dan.carpenter@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>