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.