Kernel: Make Inode::flush_metadata() return a KResult

Even if this goes nowhere yet, we have to start building an error
propagation path somewhere.

Kernel/FileSystem/DevPtsFS.cpp

@@ -145,8 +145,9 @@ KResultOr<NonnullRefPtr<Inode>> DevPtsFSInode::lookup(StringView name)
     });
 }
 
-void DevPtsFSInode::flush_metadata()
+KResult DevPtsFSInode::flush_metadata()
 {
+    return KSuccess;
 }
 
 KResult DevPtsFSInode::add_child(Inode&, const StringView&, mode_t)