net: move the hsize check to the else block in skb_segment

After commit 89319d38 ("net: Add frag_list support to skb_segment"),
it goes to process frag_list when !hsize in skb_segment(). However, when
using skb frag_list, sg normally should not be set. In this case, hsize
will be set with len right before !hsize check, then it won't go to
frag_list processing code.

So the right thing to do is move the hsize check to the else block, so
that it won't affect the !hsize check for frag_list processing.

v1->v2:
  - change to do "hsize <= 0" check instead of "!hsize", and also move
    "hsize < 0" into else block, to save some cycles, as Alex suggested.
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/skbuff.c

@@ -3894,12 +3894,8 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 		}
 
 		hsize = skb_headlen(head_skb) - offset;
-		if (hsize < 0)
-			hsize = 0;
-		if (hsize > len || !sg)
-			hsize = len;
 
-		if (!hsize && i >= nfrags && skb_headlen(list_skb) &&
+		if (hsize <= 0 && i >= nfrags && skb_headlen(list_skb) &&
 		    (skb_headlen(list_skb) == len || sg)) {
 			BUG_ON(skb_headlen(list_skb) > len);
 
@@ -3942,6 +3938,11 @@ struct sk_buff *skb_segment(struct sk_buff *head_skb,
 			skb_release_head_state(nskb);
 			__skb_push(nskb, doffset);
 		} else {
+			if (hsize > len || !sg)
+				hsize = len;
+			else if (hsize < 0)
+				hsize = 0;
+
 			nskb = __alloc_skb(hsize + doffset + headroom,
 					   GFP_ATOMIC, skb_alloc_rx_flag(head_skb),
 					   NUMA_NO_NODE);