Kernel: Implement O_DIRECT open() flag to bypass disk caches

Files opened with O_DIRECT will now bypass the disk cache in read/write
operations (though metadata operations will still hit the disk cache.)

This will allow us to test actual disk performance instead of testing
disk *cache* performance, if that's what we want. :^)

There's room for improvment here, we're very aggressively flushing any
dirty cache entries for the specific block before reading/writing that
block. This is done by walking the entire cache, which may be slow.

Kernel/FileSystem/FileDescription.cpp

@@ -293,6 +293,7 @@ void FileDescription::set_file_flags(u32 flags)
 {
     m_is_blocking = !(flags & O_NONBLOCK);
     m_should_append = flags & O_APPEND;
+    m_direct = flags & O_DIRECT;
     m_file_flags = flags;
 }