[BUG] better to ban variable length arrays

### Description / Steps to reproduce the issue

it seems that we use variable length array (VLA) here and there.
eg. https://github.com/apache/nuttx/blob/118f8271038afe24732dfc4376fb63b72e039dba/net/usrsock/usrsock_sendmsg.c#L145

VLA is a C99 feature and should not be used in the common part of the OS. i guess it's better to add `-Wvla` there.

### On which OS does this issue occur?

[OS: Mac]

### What is the version of your OS?

macOS 15.2

### NuttX Version

master

### Issue Architecture

[Arch: all]

### Issue Area

[Area: Other]

### Verification

- [x] I have verified before submitting the report.

The example is net/usrsock/usrsock_sendmsg.c at the referenced line; start by inspecting that VLA use and the surrounding build configuration. Check how the common OS is compiled and enable -Wvla there to identify the remaining scope. Done means the common part builds without VLA warnings and the affected cases are addressed.

Written by the indexing model from the issue text.