liveupdate: luo_flb: introduce File-Lifecycle-Bound global state

liveupdate: luo_flb: introduce File-Lifecycle-Bound global state

Introduce a mechanism for managing global kernel state whose lifecycle is
tied to the preservation of one or more files.  This is necessary for
subsystems where multiple preserved file descriptors depend on a single,
shared underlying resource.

An example is HugeTLB, where multiple file descriptors such as memfd and
guest_memfd may rely on the state of a single HugeTLB subsystem.
Preserving this state for each individual file would be redundant and
incorrect.  The state should be preserved only once when the first file is
preserved, and restored/finished only once the last file is handled.

This patch introduces File-Lifecycle-Bound (FLB) objects to solve this
problem.  An FLB is a global, reference-counted object with a defined set
of operations:

- A file handler (struct liveupdate_file_handler) declares a dependency
  on one or more FLBs via a new registration function,
  liveupdate_register_flb().
- When the first file depending on an FLB is preserved, the FLB's
  .preserve() callback is invoked to save the shared global state. The
  reference count is then incremented for each subsequent file.
- Conversely, when the last file is unpreserved (before reboot) or
  finished (after reboot), the FLB's .unpreserve() or .finish() callback
  is invoked to clean up the global resource.

The implementation includes:

- A new set of ABI definitions (luo_flb_ser, luo_flb_head_ser) and a
  corresponding FDT node (luo-flb) to serialize the state of all active
  FLBs and pass them via Kexec Handover.
- Core logic in luo_flb.c to manage FLB registration, reference
  counting, and the invocation of lifecycle callbacks.
- An API (liveupdate_flb_get/_incoming/_outgoing) for other kernel
  subsystems to safely access the live object managed by an FLB, both
  before and after the live update.

This framework provides the necessary infrastructure for more complex
subsystems like IOMMU, VFIO, and KVM to integrate with the Live Update
Orchestrator.

Link: https://lkml.kernel.org/r/20251218155752.3045808-5-pasha.tatashin@soleen.com
Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Alexander Graf <graf@amazon.com>
Cc: David Gow <davidgow@google.com>
Cc: David Matlack <dmatlack@google.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kees Cook <kees@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Cc: Samiullah Khawaja <skhawaja@google.com>
Cc: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>