powerpc: Define arch_is_kernel_initmem_freed() for lockdep

Under certain circumstances, we hit a warning in lockdep_register_key:

        if (WARN_ON_ONCE(static_obj(key)))
                return;

This occurs when the key falls into initmem that has since been freed
and can now be reused. This has been observed on boot, and under
memory pressure.

Define arch_is_kernel_initmem_freed(), which allows lockdep to
correctly identify this memory as dynamic.

This fixes a bug picked up by the powerpc64 syzkaller instance where
we hit the WARN via alloc_netdev_mqs.

Reported-by: Qian Cai <[email protected]>
Reported-by: ppc syzbot c/o Andrew Donnellan <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
Signed-off-by: Daniel Axtens <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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