KVM: arm64: Reimplement vgic-debug XArray iteration

The vgic-debug interface implementation uses XArray marks
(`LPI_XA_MARK_DEBUG_ITER`) to "snapshot" LPIs at the start of iteration.
This modifies global state for a read-only operation and complicates
reference counting, leading to leaks if iteration is aborted or fails.

Reimplement the iterator to use dynamic iteration logic:

- Remove `lpi_idx` from `struct vgic_state_iter`.
- Replace the XArray marking mechanism with dynamic iteration using
  `xa_find_after(..., XA_PRESENT)`.
- Wrap XArray traversals in `rcu_read_lock()`/`rcu_read_unlock()` to
  ensure safety against concurrent modifications (e.g., LPI unmapping).
- Handle potential races where an LPI is removed during iteration by
  gracefully skipping it in `show()`, rather than warning.
- Remove the unused `LPI_XA_MARK_DEBUG_ITER` definition.

This simplifies the lifecycle management of the iterator and prevents
resource leaks associated with the marking mechanism, and paves the way
for using a standard seq_file iterator.

Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260202085721.3954942-3-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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