Kernel: Expose .length() of KBufferBuilder

Author: https://github.com/MacDue
Commit: 222079cd80
Pull-request: https://github.com/SerenityOS/serenity/pull/13898
Reviewed-by: https://github.com/alimpfard

Kernel/KBufferBuilder.h

@@ -52,6 +52,11 @@ public:
         return m_buffer->bytes();
     }
 
+    size_t length() const
+    {
+        return m_size;
+    }
+
 private:
     explicit KBufferBuilder(NonnullOwnPtr<KBuffer>);