Try to reuse the lwIP pbuf in UDPEndPointImplLwIP.cpp

[gjc13]

Problem

Currently all the UDP packets are copied to fit the PacketBuffer memory model.

Proposed Solution

Check the memory layout of the lwIP pbuf and reuse it if it already fits the PacketBuffer.

[bzbarsky-apple]

Per #20923 (comment) I think we can do better than that and only copy when copying is actually needed, but just make sure we do it correctly for non-inline pbufs.

[stale]

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