[BUG] KHO handover hangs when memmap reserves PMEM

[Chris Bainbridge <chris.bainbridge@canonical.com>]

Hello,

I am reporting a regression in KHO handover.

In the most recent Ubuntu 26.10 beta mini-ISO releases (2026-09-18 onwards),
the first kernel reserves RAM for the downloaded ISO with a memmap directive
such as memmap=<size>!4G, then kexecs a second kernel. On affected kernels, the
first kernel reaches "kexec_core: Starting new kernel", but the second kernel
produces no further output and QEMU spins at 100% CPU. On unaffected kernels,
the second kernel starts and finds /dev/pmem0 with the expected "Persistent
Memory (legacy)" range in /proc/iomem.

A source bisect identified 3f2ad90060f6 ("kho: adopt radix tree for preserved
memory tracking") as the first bad commit; reverting it at the bisected
revision restored the handover. The regression appears related to KHO metadata
being accessed after the receiving kernel's memmap directive changes how the
metadata's physical range is mapped.

#regzbot introduced: 3f2ad90060f6

I have an AI-generated candidate fix that changes 22 files. I have not attached
or published it: although the AI did extensive verification on both x64 and
arm64, I cannot review or explain the complete change, so I am not submitting
it as a fix. If one of you thinks an unreviewed candidate would be useful for
further analysis, please let me know.