x86/srso: Disable the mitigation on unaffected configurations

Skip the srso cmd line parsing which is not needed on Zen1/2 with SMT
disabled and with the proper microcode applied (latter should be the
case anyway) as those are not affected.

Fixes: 5a15d8348881 ("x86/srso: Tie SBPB bit setting to microcode patch detection")
Signed-off-by: Borislav Petkov (AMD) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/kernel/cpu/bugs.c

···
 		 * IBPB microcode has been applied.
 		 */
 		if ((boot_cpu_data.x86 < 0x19) &&
-		    (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED)))
+		    (!cpu_smt_possible() || (cpu_smt_control == CPU_SMT_DISABLED))) {
 			setup_force_cpu_cap(X86_FEATURE_SRSO_NO);
+			return;
+		}
 	}
 
 	if (retbleed_mitigation == RETBLEED_MITIGATION_IBPB) {
···
 
 static ssize_t srso_show_state(char *buf)
 {
+	if (boot_cpu_has(X86_FEATURE_SRSO_NO))
+		return sysfs_emit(buf, "Not affected\n");
+
 	return sysfs_emit(buf, "%s%s\n",
 			  srso_strings[srso_mitigation],
 			  (cpu_has_ibpb_brtype_microcode() ? "" : ", no microcode"));