net: fclone allocation small optimization

After skb allocation, initial skb->fclone value is 0 (SKB_FCLONE_UNAVAILABLE)

We can replace one RMW sequence with a single OR instruction.

	movzbl 0x7e(%r13),%eax // skb->fclone = SKB_FCLONE_ORIG;
	and    $0xf3,%al
	or     $0x4,%al
	mov    %al,0x7e(%r13)
->
	or     $0x4,0x7e(%r13) // skb->fclone |= SKB_FCLONE_ORIG;

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: NipaLocal <nipa@local>

Author: edumazet

net/core/skbuff.c

@@ -723,7 +723,14 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 
 		fclones = container_of(skb, struct sk_buff_fclones, skb1);
 
-		skb->fclone = SKB_FCLONE_ORIG;
+		/* skb->fclone is a 2bits field.
+		 * Replace expensive RMW (skb->fclone = SKB_FCLONE_ORIG)
+		 * with a single OR.
+		 */
+		BUILD_BUG_ON(SKB_FCLONE_UNAVAILABLE != 0);
+		DEBUG_NET_WARN_ON_ONCE(skb->fclone != SKB_FCLONE_UNAVAILABLE);
+		skb->fclone |= SKB_FCLONE_ORIG;
+
 		refcount_set(&fclones->fclone_ref, 1);
 	}