selftests/bpf: Fix double thread join in uprobe_multi_test

selftests/bpf: Fix double thread join in uprobe_multi_test

ASAN reported a "joining already joined thread" error. The
release_child() may be called multiple times for the same struct
child.

Fix by resetting child->thread to 0 after pthread_join.

Also memset(0) static child variable in test_attach_api().

Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260223190736.649171-15-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>