bpf: Fix an issue due to uninitialized bpf_iter_task

Failure to initialize it->pos, coupled with the presence of an invalid
value in the flags variable, can lead to it->pos referencing an invalid
task, potentially resulting in a kernel panic. To mitigate this risk, it's
crucial to ensure proper initialization of it->pos to NULL.

Fixes: ac8148d957f5 ("bpf: bpf_iter_task_next: use next_task(kit->task) rather than next_task(kit->pos)")
Signed-off-by: Yafang Shao <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Acked-by: Oleg Nesterov <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/bpf/task_iter.c

···
 	BUILD_BUG_ON(__alignof__(struct bpf_iter_task_kern) !=
 					__alignof__(struct bpf_iter_task));
 
+	kit->pos = NULL;
+
 	switch (flags) {
 	case BPF_TASK_ITER_ALL_THREADS:
 	case BPF_TASK_ITER_ALL_PROCS: