Support PacketBuffer cloning for forwarding

[nhurley3]

Description

Currently, a deep copy of the packet is made when forwarding [1]. For multicast, this means that a deep copy will be made for each output interface. To make this more efficient, the packet could instead just be cloned. When cloning, the reserved header bytes needs to be able to be specified though.

[1] 

gvisor/pkg/tcpip/network/ipv4/ipv4.go

Line 710 in 47b5915

 newPkt := pkt.DeepCopyForForwarding(int(r.MaxHeaderLength())) 

Is this feature related to a specific bug?

No response

Do you have a specific solution in mind?

No response

[github-actions]

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