BPF elf examples

[Konstantin Ananyev]

> While looking in more detail at BPF use of VLA, went down a rabbit hole
> because there are no tests and no examples of loading a BPF program using
> the elf_load API's in BPF.
> 
> Tried using AI to generate tests, and as usual it sort of worked but the
> example code generated won't load or work. Is there anywhere to find working
> DPDK examples of using BPF load? 

app/test-pmd/bpf_cmd.c calls rte_bpf_eth_rx_elf_load/ rte_bpf_eth_tx_elf_load,
which in turn calls rte_bpf_elf_load().
Though it is a good point, will think can we add a UT for it too.