[$] Magic kernel functions for BPF

When programs written in BPF (the kernel's hot-loadable virtual-machine bytecode) call kernel functions (kfuncs), it may be usefulfor those functions to have additional information about the context in whichthose BPF programs are executing. Rather than requiring it to supplythat information, it would be convenient to let the BPF verifier pass thatinformation to the called function automatically. That is already possible, buta recent patch set from Ihor Solodrai would make it more ergonomic.It allows kerneldevelopers to specify that a kfunc should be passed additionalparameters inferred by the verifier, invisibly to the BPF program. Thediscussion included concerns that Solodrai's implementation was unnecessarily complex, however.