During IP fragmentation, evaluate whether we should be using WritePacket or WritePackets

**Description**

Currently when we do IP fragmentation, we create the fragment one by one and send them to the Link layer using `WritePacket`. Instead, we could create a list of fragment and send them all at once using `WritePacket`. This is faster but will cost more memory.

**Do you have a specific solution in mind?**

We could create a stack.PacketBufferList and feed it to WritePackets().