mm,ima,kexec,of: use memblock_free_late from ima_free_kexec_buffer

mm,ima,kexec,of: 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 issue.

Fixes: fee3ff99bc67 ("powerpc: Move arch independent ima kexec functions to drivers/of/kexec.c")
Cc: stable@kernel.org
Signed-off-by: Rik van Riel <riel@surriel.com>
Suggested-by: Mike Rappoport <rppt@kernel.org>
Link: https://lore.kernel.org/r/20230817135759.0888e5ef@imladris.surriel.com
Signed-off-by: Rob Herring <robh@kernel.org>