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

In the Linux kernel, the following vulnerability has been resolved:

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.