selftests/bpf: tests for iterating callbacks

selftests/bpf: tests for iterating callbacks

A set of test cases to check behavior of callback handling logic,
check if verifier catches the following situations:
- program not safe on second callback iteration;
- program not safe on zero callback iterations;
- infinite loop inside a callback.

Verify that callback logic works for bpf_loop, bpf_for_each_map_elem,
bpf_user_ringbuf_drain, bpf_find_vma.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231121020701.26440-8-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>