[$] Improved load-time checking for BPF kfuncs

The BPF verifier is charged with the
challenging task of ensuring that a BPF program is safe for the kernel to
run before that program is loaded. Among many other concerns, the verifier
must ensure that any kfuncs (kernel functions that have been exported to
BPF programs) are called with the correct parameters and from the right
context. The 'context' part of that enforcement is showing its age in ways
that are hurting performance; Juntong Deng has been working on
infrastructure to provide finer-grained control over when a kfunc can be
called.