[$] Zero-copy network transmission with io_uring

When the goal is to push bits over the network as fast as the hardware cango, any overhead hurts. The cost of copying data to be transmittedfrom user space into the kernel can be especially painful; it adds latency,takesvaluable CPU time, and can be hard on cache performance. So it isunsurprising that the developers working with io_uring, which is all about performance, haveturned their attention to zero-copy network transmission. Thispatch set from Pavel Begunkov, now in its second revision, looks to besignificantly faster than the MSG_ZEROCOPY option supported by currentkernels.