x86/cpu: Actually turn off mitigations by default for SPECULATION_MITIGATIONS=n

Initialize cpu_mitigations to CPU_MITIGATIONS_OFF if the kernel is built
with CONFIG_SPECULATION_MITIGATIONS=n, as the help text quite clearly
states that disabling SPECULATION_MITIGATIONS is supposed to turn off all
mitigations by default.

  │ If you say N, all mitigations will be disabled. You really
  │ should know what you are doing to say so.

As is, the kernel still defaults to CPU_MITIGATIONS_AUTO, which results in
some mitigations being enabled in spite of SPECULATION_MITIGATIONS=n.

Fixes: f43b9876e8 ("x86/retbleed: Add fine grained Kconfig knobs")
Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Daniel Sneddon <[email protected]>
Cc: [email protected]
Cc: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

kernel/cpu.c

@@ -3207,7 +3207,8 @@ enum cpu_mitigations {
 };
 
 static enum cpu_mitigations cpu_mitigations __ro_after_init =
-	CPU_MITIGATIONS_AUTO;
+	IS_ENABLED(CONFIG_SPECULATION_MITIGATIONS) ? CPU_MITIGATIONS_AUTO :
+						     CPU_MITIGATIONS_OFF;
 
 static int __init mitigations_parse_cmdline(char *arg)
 {