selftests/bpf: Add test for recursive attachment of tracing progs

Verify the fact that only one fentry prog could be attached to another
fentry, building up an attachment chain of limited size. Use existing
bpf_testmod as a start of the chain.

Acked-by: Jiri Olsa <olsajiri@gmail.com>
Acked-by: Song Liu <song@kernel.org>
Signed-off-by: Dmitrii Dolgov <9erthalion6@gmail.com>
Link: https://lore.kernel.org/r/20240103190559.14750-3-9erthalion6@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>