KVM: SEV: Pin guest memory for write for RECEIVE_UPDATE_DATA

Require the target guest page to be writable when pinning memory for
RECEIVE_UPDATE_DATA.  Per the SEV API, the PSP writes to guest memory:

  The result is then encrypted with GCTX.VEK and written to the memory
  pointed to by GUEST_PADDR field.

Fixes: 15fb7de1

 ("KVM: SVM: Add KVM_SEV_RECEIVE_UPDATE_DATA command")
Cc: stable@vger.kernel.org
Cc: Peter Gonda <pgonda@google.com>
Cc: Marc Orr <marcorr@google.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210914210951.2994260-2-seanjc@google.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Peter Gonda <pgonda@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>