Kernel: Add Inode::truncate(size).

- Use this to implement the O_TRUNC open flag.
- Fix creat() to pass O_CREAT | O_TRUNC | O_WRONLY.
- Make sure we truncate wherever appropriate.

Kernel/FileSystem.h

@@ -103,6 +103,7 @@ public:
     virtual size_t directory_entry_count() const = 0;
     virtual KResult chmod(mode_t) = 0;
     virtual KResult chown(uid_t, gid_t) = 0;
+    virtual KResult truncate(int) { return KSuccess; }
 
     LocalSocket* socket() { return m_socket.ptr(); }
     const LocalSocket* socket() const { return m_socket.ptr(); }