x86/speculation: Fix SPEC_CTRL write on SMT state change

If the SMT state changes, SSBD might get accidentally disabled.  Fix
that.
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>

arch/x86/kernel/cpu/bugs.c

@@ -1458,7 +1458,8 @@ static void __init spectre_v2_select_mitigation(void)
 
 static void update_stibp_msr(void * __unused)
 {
-	write_spec_ctrl_current(x86_spec_ctrl_base, true);
+	u64 val = spec_ctrl_current() | (x86_spec_ctrl_base & SPEC_CTRL_STIBP);
+	write_spec_ctrl_current(val, true);
 }
 
 /* Update x86_spec_ctrl_base in case SMT state changed. */