KVM: arm64: Split kvm_pgtable_stage2_destroy()

KVM: arm64: Split kvm_pgtable_stage2_destroy()

Split kvm_pgtable_stage2_destroy() into two:
  - kvm_pgtable_stage2_destroy_range(), that performs the
    page-table walk and free the entries over a range of addresses.
  - kvm_pgtable_stage2_destroy_pgd(), that frees the PGD.

This refactoring enables subsequent patches to free large page-tables
in chunks, calling cond_resched() between each chunk, to yield the
CPU as necessary.

Existing callers of kvm_pgtable_stage2_destroy(), that probably cannot
take advantage of this (such as nVMHE), will continue to function as is.

Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Suggested-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250820162242.2624752-2-rananta@google.com
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>