x86/bugs: Do IBPB fallback check only once

When booting with retbleed=auto, if the kernel wasn't built with
CONFIG_CC_HAS_RETURN_THUNK, the mitigation falls back to IBPB.  Make
sure a warning is printed in that case.  The IBPB fallback check is done
twice, but it really only needs to be done once.
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>

arch/x86/kernel/cpu/bugs.c

@@ -891,18 +891,13 @@ static void __init retbleed_select_mitigation(void)
 	case RETBLEED_CMD_AUTO:
 	default:
 		if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
-		    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON) {
-
-			if (IS_ENABLED(CONFIG_RETPOLINE) &&
-			    IS_ENABLED(CONFIG_CC_HAS_RETURN_THUNK))
+		    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)
 			retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
-			else
-				retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
-		}
 
 		/*
-		 * The Intel mitigation (IBRS) was already selected in
-		 * spectre_v2_select_mitigation().
+		 * The Intel mitigation (IBRS or eIBRS) was already selected in
+		 * spectre_v2_select_mitigation().  'retbleed_mitigation' will
+		 * be set accordingly below.
 		 */
 
 		break;