selftests/bpf: Add test for trie_get_next_key()

Add a test for out-of-bounds write in trie_get_next_key() when a full
path from root to leaf exists and bpf_map_get_next_key() is called
with the leaf node. It may crashes the kernel on failure, so please
run in a VM.

Signed-off-by: Byeonguk Jeong <jungbu2855@gmail.com>
Acked-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/Zxx4ep78tsbeWPVM@localhost.localdomain


Signed-off-by: Alexei Starovoitov <ast@kernel.org>