Squash to "bpf: Add mptcp_subflow bpf_iter"

Address Martin's comments in v1:

- bpf_iter_mptcp_subflow_new returns -EINVAL when msk socket lock isn't
  held.

Signed-off-by: Geliang Tang <[email protected]>

Author: Geliang Tang

net/mptcp/bpf.c

@@ -252,7 +252,8 @@ bpf_iter_mptcp_subflow_new(struct bpf_iter_mptcp_subflow *it,
 	if (!msk)
 		return -EINVAL;
 
-	msk_owned_by_me(msk);
+	if (!lockdep_sock_is_held((const struct sock *)msk))
+		return -EINVAL;
 
 	kit->pos = &msk->conn_list;
 	return 0;