slab: Remove __malloc attribute from realloc functions

The __malloc attribute should not be applied to "realloc" functions, as
the returned pointer may alias the storage of the prior pointer. Instead
of splitting __malloc from __alloc_size, which would be a huge amount of
churn, just create __realloc_size for the few cases where it is needed.

Thanks to Geert Uytterhoeven <[email protected]> for reporting build
failures with gcc-8 in earlier version which tried to remove the #ifdef.
While the "alloc_size" attribute is available on all GCC versions, I
forgot that it gets disabled explicitly by the kernel in GCC < 9.1 due
to misbehaviors. Add a note to the compiler_attributes.h entry for it.

Cc: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Roman Gushchin <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>

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