SLAB_FREELIST_HARDENED can protect from freelist overwrite attacks with
really small overhead.

It works best with the SLUB allocator, so make SLUB the default by
removing SLAB=y.

      total   used           free           shared        buff/cache    available
SLAB  425596  44065.3+/-220  311099+/-3800  14864+/-3900  70432+/-3700  352767+/-3900

SLUB  425592  44225.3+/-280  313275+/-600   12132+/-3.3   68092+/-530   355295+/-280

These figures are the average memory usage from three boots of each
option in qemu, running the Romulus userspace. The output is from
free(1), reported in kilobytes.
Signed-off-by: NJoel Stanley <joel@jms.id.au>
Reviewed-by: NAndrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20210819065203.2620911-2-joel@jms.id.auSigned-off-by: NJoel Stanley <joel@jms.id.au>