lib/test_kho: use kho_preserve_vmalloc instead of storing addresses in fdt

lib/test_kho: use kho_preserve_vmalloc instead of storing addresses in fdt

KHO test stores physical addresses of the preserved folios directly in
fdt.  Use kho_preserve_vmalloc() instead of it and kho_restore_vmalloc()
to retrieve the addresses after kexec.

This makes the test more scalable from one side and adds tests coverage
for kho_preserve_vmalloc() from the other.

Link: https://lkml.kernel.org/r/20250921054458.4043761-5-rppt@kernel.org
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Changyuan Lyu <changyuanl@google.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>