mm, slub: splice cpu and page freelists in deactivate_slab()

In deactivate_slab() we currently move all but one objects on the cpu
freelist to the page freelist one by one using the costly cmpxchg_double()
operation.  Then we unfreeze the page while moving the last object on page
freelist, with a final cmpxchg_double().

This can be optimized to avoid the cmpxchg_double() per object.  Just
count the objects on cpu freelist (to adjust page->inuse properly) and
also remember the last object in the chain.  Then splice page->freelist to
the last object and effectively add the whole cpu freelist to
page->freelist while unfreezing the page, with a single cmpxchg_double().

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Vlastimil Babka <[email protected]>
Reviewed-by: Jann Horn <[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]>

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