mm/slub: Improve redzone check and zeroing for krealloc()

For current krealloc(), one problem is its caller doesn't pass the old
request size, say the object is 64 bytes kmalloc one, but caller may
only requested 48 bytes. Then when krealloc() shrinks or grows in the
same object, or allocate a new bigger object, it lacks this 'original
size' information to do accurate data preserving or zeroing (when
__GFP_ZERO is set).

Thus with slub debug redzone and object tracking enabled, parts of the
object after krealloc() might contain redzone data instead of zeroes,
which is violating the __GFP_ZERO guarantees. Good thing is in this
case, kmalloc caches do have this 'orig_size' feature. So solve the
problem by utilize 'org_size' to do accurate data zeroing and preserving.

[Thanks to syzbot and V, Narasimhan for discovering kfence and big
 kmalloc related issues in early patch version]

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>