selftests/bpf: Test tail call counting with bpf2bpf and data on stack

Cover the case when tail call count needs to be passed from BPF function to
BPF function, and the caller has data on stack. Specifically when the size
of data allocated on BPF stack is not a multiple on 8.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220616162037.535469-3-jakub@cloudflare.com