Claude review: iommu/io-pgtable: introduce iova_to_phys_length in io_pgtable_ops

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

Clean mechanical changes to ARM LPAE, ARM v7s, and DART backends.

**v7s concern** -- the `blk_size` calculation:
```c
blk_size = ~mask + 1U;
```
When `mask` has been multiplied by `ARM_V7S_CONT_PAGES` (16), this gives the contiguous block size. The `1U` suffix means unsigned 32-bit arithmetic. Since `mask` is `u32`, this is fine, but on a 32-bit ARM with a 4GB mask this could theoretically overflow. In practice the masks used here are well below that, so this is OK.

**Selftest changes in v7s**: The selftests switch from `ops->iova_to_phys()` to `ops->iova_to_phys_length(ops, ..., NULL)` but none of them actually test the new `mapped_length` output. This is a missed opportunity -- it would be very valuable to add selftest coverage verifying that the returned `mapped_length` matches the expected page size for each mapping.

**DART backend**: Using `data->iop.cfg.pgsize_bitmap` as `mapped_length` is questionable:
```c
pgsize = data->iop.cfg.pgsize_bitmap;
if (mapped_length)
    *mapped_length = pgsize;
```
`pgsize_bitmap` is a bitmask of *supported* page sizes, not the actual page size of the mapping. For DART (Apple), the bitmap happens to be a single bit (one fixed page size), so treating it as a size works by accident. This should at minimum have a comment explaining why it's correct, or better, use `__ffs(pgsize_bitmap)` / `BIT()` to be explicit. The v2 cover letter doesn't mention fixing this.

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