x86/mm, kexec, ima: Use memblock_free_late() from ima_free_kexec_buffer()

The code calling ima_free_kexec_buffer() runs long after the memblock
allocator has already been torn down, potentially resulting in a use
after free in memblock_isolate_range().

With KASAN or KFENCE, this use after free will result in a BUG
from the idle task, and a subsequent kernel panic.

Switch ima_free_kexec_buffer() over to memblock_free_late() to avoid
that bug.

Fixes: fee3ff99 ("powerpc: Move arch independent ima kexec functions to drivers/of/kexec.c")
Suggested-by: Mike Rappoport <rppt@kernel.org>
Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230817135558.67274c83@imladris.surriel.com

arch/x86/kernel/setup.c

@@ -358,15 +358,11 @@ static void __init add_early_ima_buffer(u64 phys_addr)
 #if defined(CONFIG_HAVE_IMA_KEXEC) && !defined(CONFIG_OF_FLATTREE)
 int __init ima_free_kexec_buffer(void)
 {
-	int rc;
-
 	if (!ima_kexec_buffer_size)
 		return -ENOENT;
 
-	rc = memblock_phys_free(ima_kexec_buffer_phys,
-				ima_kexec_buffer_size);
-	if (rc)
-		return rc;
+	memblock_free_late(ima_kexec_buffer_phys,
+			   ima_kexec_buffer_size);
 
 	ima_kexec_buffer_phys = 0;
 	ima_kexec_buffer_size = 0;