refcount: Report UAF for refcount_sub_and_test(0) when counter==0

When a reference counter is at zero and refcount_sub_and_test() is invoked
to subtract zero, the function accepts this request without any warning and
returns true. This behavior does not seem ideal because the counter being
already at zero indicates a use-after-free. Furthermore, returning true by
refcount_sub_and_test() in this case potentially results in a double-free
done by its caller.

Modify the underlying function __refcount_sub_and_test() to warn about this
case as a use-after-free and have it return false to avoid the potential
double-free.

Signed-off-by: Petr Pavlu <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Kees Cook <[email protected]>

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