bpf: allow access to skb->len from offloads

Since we are now doing strict checking of what offloads
may access, make sure skb->len is on that list.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/filter.c

@@ -3738,6 +3738,8 @@ tc_cls_act_is_valid_access_analyzer(int off, int size,
 				    struct bpf_insn_access_aux *info)
 {
 	switch (off) {
+	case offsetof(struct sk_buff, len):
+		return true;
 	case offsetof(struct sk_buff, data):
 		info->reg_type = PTR_TO_PACKET;
 		return true;