selftests/bpf: Add test for prealloc_lru_pop bug

selftests/bpf: Add test for prealloc_lru_pop bug

Add a regression test to check against invalid check_and_init_map_value
call inside prealloc_lru_pop.

The kptr should not be reset to NULL once we set it after deleting the
map element. Hence, we trigger a program that updates the element
causing its reuse, and checks whether the unref kptr is reset or not.
If it is, prealloc_lru_pop does an incorrect check_and_init_map_value
call and the test fails.

Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20220809213033.24147-4-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>