kexec: introduce is_kho_boot()

kexec: introduce is_kho_boot()

Patch series "efi: Fix EFI boot with kexec handover (KHO)", v3.

This patch series fixes a kernel panic that occurs when booting with both
EFI and KHO (Kexec HandOver) enabled.

The issue arises because EFI's `reserve_regions()` clears all memory
regions with `memblock_remove(0, PHYS_ADDR_MAX)` before rebuilding them
from EFI data.  This destroys KHO scratch regions that were set up early
during device tree scanning, causing a panic as the kernel has no valid
memory regions for early allocations.

The first patch introduces `is_kho_boot()` to allow early boot components
to reliably detect if the kernel was booted via KHO-enabled kexec.  The
existing `kho_is_enabled()` only checks the command line and doesn't
verify if an actual KHO FDT was passed.

The second patch modifies EFI's `reserve_regions()` to selectively remove
only non-KHO memory regions when KHO is active, preserving the critical
scratch regions while still allowing EFI to rebuild its memory map.

This patch (of 3):

During early initialisation, after a kexec, other components, like EFI
need to know if a KHO enabled kexec is performed.  The `kho_is_enabled`
function is not enough as in the early stages, it only reflects whether
the cmdline has KHO enabled, not if an actual KHO FDT exists.

Extend the KHO API with `is_kho_boot()` to provide a way for components to
check if a KHO enabled kexec is performed.

Link: https://lkml.kernel.org/r/cover.1755721529.git.epetron@amazon.de
Link: https://lkml.kernel.org/r/7dc6674a76bf6e68cca0222ccff32427699cc02e.1755721529.git.epetron@amazon.de
Signed-off-by: Evangelos Petrongonas <epetron@amazon.de>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Pratyush Yadav <pratyush@kernel.org>
Cc: Alexander Graf <graf@amazon.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: Changyuan Lyu <changyuanl@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>