config: enable SLAB_FREELIST_HARDENED (bsc#1127808)
    
    Enable SLAB_FREELIST_HARDENED on all architectures. This obscures the
    free object pointer on a per-cache basis making it more difficult to
    locate kernel objects via exploits probing the cache metadata.
    
    This change was requested by the upstream openSUSE community to make
    the kernel more resistent to slab freelist attacks. Tests conducted
    by the kernel performance teams confirmed that the performance impact
    is detectable but negligible.