bpf: Provide warning message when BPF filesystem is not mounted
bpf: Provide warning message when BPF filesystem is not mounted

It was not possible to determine based on log files when the BPF filesystem was
required to be mounted or not.

Signed-off-by: Thomas Graf <5f50a84c1fa3bcff146405017f36aec1a10a9e38@cilium.io>

michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,cilium-team/cilium,tklauser/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium-team/cilium,michi-covalent/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium