[$] Improved load-time checking for BPF kfuncs

The BPF verifier is charged with thechallenging task of ensuring that a BPF program is safe for the kernel torun before that program is loaded. Among many other concerns, the verifiermust ensure that any kfuncs (kernel functions that have been exported toBPF programs) are called with the correct parameters and from the rightcontext. The "context" part of that enforcement is showing its age in waysthat are hurting performance; Juntong Deng has been working oninfrastructure to provide finer-grained control over when a kfunc can becalled.