net: Cleanup skb cloning by adding SKB_FCLONE_FREE

SKB_FCLONE_UNAVAILABLE has overloaded meaning depending on type of skb.
1: If skb is allocated from head_cache, it indicates fclone is not available.
2: If skb is a companion fclone skb (allocated from fclone_cache), it indicates
it is available to be used.

To avoid confusion for case 2 above, this patch  replaces
SKB_FCLONE_UNAVAILABLE with SKB_FCLONE_FREE where appropriate. For fclone
companion skbs, this indicates it is free for use.

SKB_FCLONE_UNAVAILABLE will now simply indicate skb is from head_cache and
cannot / will not have a companion fclone.

Signed-off-by: Vijay Subramanian <subramanian.vijay@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 7c5036d..3a5ec76 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -339,9 +339,10 @@
 
 
 enum {
-	SKB_FCLONE_UNAVAILABLE,
-	SKB_FCLONE_ORIG,
-	SKB_FCLONE_CLONE,
+	SKB_FCLONE_UNAVAILABLE,	/* skb has no fclone (from head_cache) */
+	SKB_FCLONE_ORIG,	/* orig skb (from fclone_cache) */
+	SKB_FCLONE_CLONE,	/* companion fclone skb (from fclone_cache) */
+	SKB_FCLONE_FREE,	/* this companion fclone skb is available */
 };
 
 enum {