bpf: reserve flags in bpf_skb_net_shrink

The ENCAP flags in bpf_skb_adjust_room are ignored on decap with
bpf_skb_net_shrink. Reserve these bits for future use.

Fixes: 868d523535 ("bpf: add bpf_skb_adjust_room encap flags")
Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/filter.c

@@ -3069,6 +3069,9 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff,
 {
 	int ret;
 
+	if (flags & ~BPF_F_ADJ_ROOM_FIXED_GSO)
+		return -EINVAL;
+
 	if (skb_is_gso(skb) && !skb_is_gso_tcp(skb)) {
 		/* udp gso_size delineates datagrams, only allow if fixed */
 		if (!(skb_shinfo(skb)->gso_type & SKB_GSO_UDP_L4) ||