os: fix race condition in readdir by atomically initializing dirinfo

[Mehrbod2002]

This change ensures that dirinfo in the File struct is initialized atomically,
avoiding redundant allocations when multiple goroutines access it concurrently.
Instead of creating separate buffers, we now use CompareAndSwap to guarantee
thread-safe initialization and reduce unnecessary memory usage.

Although this is not a strict race condition, the update enhances efficiency by
eliminating duplicate allocations and ensuring safer concurrent access.

Fixes #71496.

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