mm/slab_common.c: mark kmalloc machinery as __ro_after_init

mm/slab_common.c: mark kmalloc machinery as __ro_after_init

kmalloc caches aren't relocated after being set up neither does
"size_index" array.

Link: http://lkml.kernel.org/r/20180226203519.GA6886@avx2
Signed-off-by: Alexey Dobriyan <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>