netfilter: flowtable: check for maximum number of encapsulations in bridge vlan

Add a sanity check to skip path discovery if the maximum number of
encapsulation is reached. While at it, check for underflow too.

Fixes: 26267bf9bb57 ("netfilter: flowtable: bridge vlan hardware offload and switchdev")
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nft_flow_offload.c

···
 				info->ingress_vlans |= BIT(info->num_encaps - 1);
 				break;
 			case DEV_PATH_BR_VLAN_TAG:
+				if (info->num_encaps >= NF_FLOW_TABLE_ENCAP_MAX) {
+					info->indev = NULL;
+					break;
+				}
 				info->encap[info->num_encaps].id = path->bridge.vlan_id;
 				info->encap[info->num_encaps].proto = path->bridge.vlan_proto;
 				info->num_encaps++;
 				break;
 			case DEV_PATH_BR_VLAN_UNTAG:
-				info->num_encaps--;
+				if (WARN_ON_ONCE(info->num_encaps-- == 0)) {
+					info->indev = NULL;
+					break;
+				}
 				break;
 			case DEV_PATH_BR_VLAN_KEEP:
 				break;