Currently, a packet being read is split into header and data buffers. Change to a single buffer to simplify logic in drivers, especially for those that receive packets with variable sized headers. This change also implements the pending send size functionality for the receive buffer. Change-Id: Ic4a3698fcfe813fbcf4fd9c1941102b5adaeb3b2 Reviewed-on: https://review.haiku-os.org/c/haiku/+/10418 Tested-by: Commit checker robot <[email protected]> Reviewed-by: Jérôme Duval <[email protected]>