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.

Link: https://github.com/linuxppc/issues/issues/284
Link: https://lore.kernel.org/linuxppc-dev/87ef0vpfbc.fsf@mpe.ellerman.id.au/ (local)
Reported-by: Qian Cai <redacted>
Reported-by: ppc syzbot c/o Andrew Donnellan <redacted>
Commit-message-by: Daniel Axtens [off-list ref]
<mpe signoff here>

---

The ppc64 syzkaller link is probably not stable enough to go into
the git history forever, but fwiw:
https://syzkaller-ppc64.appspot.com/bug?id=cfdf75cd985012d0124cd41e6fa095d33e7d0f6b

---
 arch/powerpc/include/asm/sections.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/arch/powerpc/include/asm/sections.h b/arch/powerpc/include/asm/sections.h
index 5a9b6eb651b6..d19871763ed4 100644
--- a/arch/powerpc/include/asm/sections.h
+++ b/arch/powerpc/include/asm/sections.h

@@ -5,8 +5,22 @@
 
 #include <linux/elf.h>
 #include <linux/uaccess.h>
+
+#define arch_is_kernel_initmem_freed arch_is_kernel_initmem_freed
+
 #include <asm-generic/sections.h>
 
+extern bool init_mem_is_free;
+
+static inline int arch_is_kernel_initmem_freed(unsigned long addr)
+{
+	if (!init_mem_is_free)
+		return 0;
+
+	return addr >= (unsigned long)__init_begin &&
+		addr < (unsigned long)__init_end;
+}
+
 extern char __head_end[];
 
 #ifdef __powerpc64__