bpf, devmap: Compute proper xdp_frame len redirecting frames

Even if it is currently forbidden to XDP_REDIRECT a multi-frag xdp_frame into
a devmap, compute proper xdp_frame length in __xdp_enqueue and is_valid_dst
routines running xdp_get_frame_len().
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/894d99c01139e921bdb6868158ff8e67f661c072.1658596075.git.lorenzo@kernel.org

kernel/bpf/devmap.c

@@ -477,7 +477,7 @@ static inline int __xdp_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
 	if (!dev->netdev_ops->ndo_xdp_xmit)
 		return -EOPNOTSUPP;
 
-	err = xdp_ok_fwd_dev(dev, xdpf->len);
+	err = xdp_ok_fwd_dev(dev, xdp_get_frame_len(xdpf));
 	if (unlikely(err))
 		return err;
 
@@ -536,7 +536,7 @@ static bool is_valid_dst(struct bpf_dtab_netdev *obj, struct xdp_frame *xdpf)
 	    !obj->dev->netdev_ops->ndo_xdp_xmit)
 		return false;
 
-	if (xdp_ok_fwd_dev(obj->dev, xdpf->len))
+	if (xdp_ok_fwd_dev(obj->dev, xdp_get_frame_len(xdpf)))
 		return false;
 
 	return true;