x86/hyperv: Fix undefined reference to isolation_type_en_snp without CONFIG_HYPERV

When CONFIG_HYPERV is not set, arch/x86/hyperv/ivm.c is not built (see
arch/x86/Kbuild), so 'isolation_type_en_snp' in the ivm.c is not defined,
and this failure happens:

ld: arch/x86/kernel/cpu/mshyperv.o: in function `ms_hyperv_init_platform':
arch/x86/kernel/cpu/mshyperv.c:417: undefined reference to `isolation_type_en_snp'

Fix the failure by testing hv_get_isolation_type() and
ms_hyperv.paravisor_present for a fully enlightened SNP VM: when
CONFIG_HYPERV is not set, hv_get_isolation_type() is defined as a
static inline function that always returns HV_ISOLATION_TYPE_NONE
(see include/asm-generic/mshyperv.h), so the compiler won't generate any
code for the ms_hyperv.paravisor_present and static_branch_enable().
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Closes: https://lore.kernel.org/lkml/b4979997-23b9-0c43-574e-e4a3506500ff@amd.com/
Fixes: d6e2d652 ("x86/hyperv: Add sev-snp enlightened guest static key")
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/20230823032008.18186-1-decui@microsoft.com

arch/x86/kernel/cpu/mshyperv.c

@@ -413,10 +413,11 @@ static void __init ms_hyperv_init_platform(void)
 			ms_hyperv.isolation_config_a, ms_hyperv.isolation_config_b);
 
 
-		if (cc_platform_has(CC_ATTR_GUEST_SEV_SNP)) {
-			static_branch_enable(&isolation_type_en_snp);
-		} else if (hv_get_isolation_type() == HV_ISOLATION_TYPE_SNP) {
-			static_branch_enable(&isolation_type_snp);
+		if (hv_get_isolation_type() == HV_ISOLATION_TYPE_SNP) {
+			if (ms_hyperv.paravisor_present)
+				static_branch_enable(&isolation_type_snp);
+			else
+				static_branch_enable(&isolation_type_en_snp);
 		}
 	}