Claude review: io_uring/rsrc: extend buffer update

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

Patch Review

Extends the buffer registration to support the new `io_uring_regbuf_desc` format via the `IORING_RSRC_UPDATE_EXTENDED` flag.

**UAPI concern: `IORING_RSRC_UPDATE_EXTENDED = 1U << 1`**

The flag uses bit 1 (value 2), skipping bit 0. Was `1U << 0` intentionally reserved? If not, this should be `1U << 0` for cleaner flag space usage. If so, a comment explaining what bit 0 is reserved for would be helpful.

**Repurposing `resv` field as `flags`**

```c
struct io_uring_rsrc_update2 {
    __u32 offset;
-   __u32 resv;
+   __u32 flags;
```

The old code checked `up.resv || up.resv2` to reject non-zero reserved fields. By renaming `resv` to `flags`, the check is relaxed — the file update path now only checks `up.flags` (and rejects non-zero), while the buffer update path accepts `IORING_RSRC_UPDATE_EXTENDED`. The `resv2` check remains. This is backward-compatible since old userspace always passed 0 for `resv`, but it's worth verifying there are no out-of-tree users relying on the `resv` rejection.

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