x86/bugs: Fix reporting of LFENCE retpoline

The LFENCE retpoline mitigation is not secure but the kernel prints
inconsistent messages about this fact.  The dmesg log says 'Mitigation:
LFENCE', implying the system is mitigated.  But sysfs reports 'Vulnerable:
LFENCE' implying the system (correctly) is not mitigated.

Fix this by printing a consistent 'Vulnerable: LFENCE' string everywhere
when this mitigation is selected.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/20250915134706.3201818-1-david.kaplan@amd.com