fix: return os.ErrClosed on double Close() in MemMapFs

[Yanhu007]

Fixes #567

Problem

mem.File.Close() silently succeeds when called multiple times, while os.File.Close() returns an error (os.ErrClosed). This inconsistency causes bugs to go undetected in tests using MemMapFs that only surface in production with OsFs.

Fix

Check the closed flag under the lock before proceeding. If already closed, return os.ErrClosed to match os.File behavior.

Testing

All existing tests pass.

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