xdp: linearize skb in netif_receive_generic_xdp()

In netif_receive_generic_xdp(), it is necessary to linearize all
nonlinear skb. However, in current implementation, skb with
troom <= 0 are not linearized. This patch fixes this by calling
skb_linearize() for all nonlinear skb.

Fixes: de8f3a83 ("bpf: add meta pointer for direct access")
Signed-off-by: Song Liu <songliubraving@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/core/dev.c

@@ -3904,7 +3904,7 @@ static u32 netif_receive_generic_xdp(struct sk_buff *skb,
 				     hroom > 0 ? ALIGN(hroom, NET_SKB_PAD) : 0,
 				     troom > 0 ? troom + 128 : 0, GFP_ATOMIC))
 			goto do_drop;
-		if (troom > 0 && __skb_linearize(skb))
+		if (skb_linearize(skb))
 			goto do_drop;
 	}