KVM: SVM: Force sev_es_host_save_area() to be inlined (for noinstr usage)

Force sev_es_host_save_area() to be always inlined, as it's used in the
low level VM-Enter/VM-Exit path, which is non-instrumentable.

  vmlinux.o: warning: objtool: svm_vcpu_enter_exit+0xb0: call to
    sev_es_host_save_area() leaves .noinstr.text section
  vmlinux.o: warning: objtool: svm_vcpu_enter_exit+0xbf: call to
    sev_es_host_save_area.isra.0() leaves .noinstr.text section

Fixes: c92be2f ("KVM: SVM: Save/restore non-volatile GPRs in SEV-ES VMRUN via host save area")
Reported-by: Borislav Petkov <bp@alien8.de>
Tested-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240617210432.1642542-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/svm.c

@@ -1495,7 +1495,7 @@ static void svm_vcpu_free(struct kvm_vcpu *vcpu)
 	__free_pages(virt_to_page(svm->msrpm), get_order(MSRPM_SIZE));
 }
 
-static struct sev_es_save_area *sev_es_host_save_area(struct svm_cpu_data *sd)
+static __always_inline struct sev_es_save_area *sev_es_host_save_area(struct svm_cpu_data *sd)
 {
 	return page_address(sd->save_area) + 0x400;
 }