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/VirtualFileSystem.h

@@ -24,6 +24,7 @@
 #define O_DIRECTORY 00200000
 #define O_NOFOLLOW 00400000
 #define O_CLOEXEC 02000000
+#define O_DIRECT 04000000
 #define O_NOFOLLOW_NOERROR 0x4000000
 
 class Custody;