src/mongo/db/storage/paths.h and .cpp contains code that is only consumed by mmap, with the exception of flushMyDirectory(). flushMyDirectory() is used by the posix implementation of StorageEngineLockFile::writePid().
We should move this code into the mmap subdirectory. The flushMyDirectory() can move along with the rest of paths.h into mmap, and a copy can be placed directly into storage_engine_lock_file_posix.cpp.