Probe matcher: list kfuncs only if supported
Probe matcher: list kfuncs only if supported

It may happen that even though BTF is available, kernel does not have
kfuncs. In such a case, we must not allow kfuncs in probe matcher as it
may cause problems e.g. when listing all available probes.

iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace