bpf: Fix a typo in comment for DFS algorithm

There is a typo in comment for DFS algorithm in bpf/verifier.c. The top
element should not be popped until all its neighbors have been checked.
Fix it.

Fixes: 475fb78f ("bpf: verifier (add branch/goto checks)")
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221027034458.2925218-1-xukuohai@huaweicloud.com

kernel/bpf/verifier.c

@@ -10670,7 +10670,7 @@ static int check_return_code(struct bpf_verifier_env *env)
  * 3      let S be a stack
  * 4      S.push(v)
  * 5      while S is not empty
- * 6            t <- S.pop()
+ * 6            t <- S.peek()
  * 7            if t is what we're looking for:
  * 8                return t
  * 9            for all edges e in G.adjacentEdges(t) do