kho: only fill kimage if KHO is finalized

kho_fill_kimage() only checks for KHO being enabled before filling in the
FDT to the image. KHO being enabled does not mean that the kernel has
data to hand over. That happens when KHO is finalized.

When a kexec is done with KHO enabled but not finalized, the FDT page is
allocated but not initialized. FDT initialization happens after finalize.
This means the KHO segment is filled in but the FDT contains garbage
data.

This leads to the below error messages in the next kernel:

[ 0.000000] KHO: setup: handover FDT (0x10116b000) is invalid: -9
[ 0.000000] KHO: disabling KHO revival: -22

There is no problem in practice, and the next kernel boots and works fine.
But this still leads to misleading error messages and garbage being
handed over.

Only fill in KHO segment when KHO is finalized. When KHO is not enabled,
the debugfs interface is not created and there is no way to finalize it
anyway. So the check for kho_enable is not needed, and kho_out.finalize
alone is enough.

Link: https://lkml.kernel.org/r/20250918170617.91413-1-pratyush@kernel.org
Fixes: 3bdecc3c93f9 ("kexec: add KHO support to kexec file loads")
Signed-off-by: Pratyush Yadav <pratyush@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Changyuan Lyu <changyuanl@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>