bpf: Allow bpf_current_task_under_cgroup in interrupt

Currently, the bpf_current_task_under_cgroup helper has a check where if
the BPF program is running in_interrupt(), it will return -EINVAL. This
prevents the helper to be used in many useful scenarios, particularly
BPF programs attached to Perf Events.

This commit removes the check. Tested a few NMI (Perf Event) and some
softirq context, the helper returns the correct result.
Signed-off-by: Teng Qin <qinteng@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/trace/bpf_trace.c

@@ -475,8 +475,6 @@ BPF_CALL_2(bpf_current_task_under_cgroup, struct bpf_map *, map, u32, idx)
 	struct bpf_array *array = container_of(map, struct bpf_array, map);
 	struct cgroup *cgrp;
 
-	if (unlikely(in_interrupt()))
-		return -EINVAL;
 	if (unlikely(idx >= array->map.max_entries))
 		return -E2BIG;