Claude review: selftests/net: ncdevmem: add -b option to set rx-buf-size on bind

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

Patch Review

The selftest C changes are clean and well-validated.

**Observations:**

1. **Good input validation** -- power-of-2, >= PAGE_SIZE, <= 2MB. The 2MB cap matches the MFD_HUGETLB + MFD_HUGE_2MB semantics and is well-documented in the commit message.

2. **Hugepage fallback message is helpful:**

```c
pr_err("[skip,no-memfd%s]",
       (memfd_flags & MFD_HUGETLB) ? " (need hugepages)" : "");
```

Good UX for CI environments without hugepages allocated.

3. **Size rounding only for hugetlb:**

```c
if (memfd_flags & MFD_HUGETLB) {
    size = roundup(size, MB(2));
    ctx->size = size;
}
```

This correctly updates `ctx->size` after rounding. The `roundup` macro comes from `<sys/param.h>` which was added. Note that `roundup` is a BSD extension -- this is fine for selftests but wouldn't be appropriate in kernel code.

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