Claude review: misc: fastrpc: Move fdlist to invoke context structure

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

Patch Review

**Status: Looks good.**

This moves the `fdlist` pointer from being locally computed in both `fastrpc_get_args()` and `fastrpc_put_args()` into the `struct fastrpc_invoke_ctx`. This eliminates redundant computation and is also necessary for patch 4 to calculate `poll_addr` relative to the end of the metadata buffer.

One subtle correctness point: the old code in `fastrpc_put_args()` computed `fdlist` as:
```c
fdlist = (uint64_t *)(pages + inbufs + outbufs + handles);
```
while the new code in `fastrpc_get_args()` computes it as:
```c
ctx->fdlist = (u64 *)(pages + ctx->nscalars);
```

These are equivalent because `nscalars = inbufs + outbufs + handles` (see `REMOTE_SCALARS_LENGTH`), so this is correct and also simpler.

Has R-b from Dmitry Baryshkov. No issues.

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