assertion failure for direct I/O writes together with mmap reads

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### Expected behavior
It should either return an unsupported error message or allow the configuration.

### Actual behavior
Triggers an assertion:
```
db_stress: env/io_posix.cc:449: rocksdb::PosixMmapReadableFile::PosixMmapReadableFile(int, const string&, void*, size_t, const rocksdb::EnvOptions&): Assertion `!options.use_direct_reads' failed.
Aborted (core dumped)
```

### Steps to reproduce the behavior
Build `db_stress` with assertions enabled, then run
`./db_stress -use_direct_io_for_flush_and_compaction=true -mmap_read=true`

Start in env/io_posix.cc at the PosixMmapReadableFile assertion shown in the report, then run db_stress with -use_direct_io_for_flush_and_compaction=true -mmap_read=true and assertions enabled. Trace how these options are configured together. Done means the combination returns an unsupported error instead of aborting, or works without triggering the assertion.

Written by the indexing model from the issue text.