mm, slub: introduce static key for slub_debug()

One advantage of CONFIG_SLUB_DEBUG is that a generic distro kernel can be
built with the option enabled, but it's inactive until simply enabled on
boot, without rebuilding the kernel.  With a static key, we can further
eliminate the overhead of checking whether a cache has a particular debug
flag enabled if we know that there are no such caches (slub_debug was not
enabled during boot).  We use the same mechanism also for e.g.
page_owner, debug_pagealloc or kmemcg functionality.

This patch introduces the static key and makes the general check for
per-cache debug flags kmem_cache_debug() use it.  This benefits several
call sites, including (slow path but still rather frequent) __slab_free().
The next patches will add more uses.

Signed-off-by: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Acked-by: Christoph Lameter <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Vijayanand Jitta <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Pekka Enberg <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Linus Torvalds <[email protected]>

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