net: introduce skb_list_walk_safe for skb segment walking

commit dcfea72e79b0aa7a057c8f6024169d86a1bbc84b upstream.

As part of the continual effort to remove direct usage of skb->next and
skb->prev, this patch adds a helper for iterating through the
singly-linked variant of skb lists, which are used for lists of GSO
packet. The name "skb_list_..." has been chosen to match the existing
function, "kfree_skb_list, which also operates on these singly-linked
lists, and the "..._walk_safe" part is the same idiom as elsewhere in
the kernel.

This patch removes the helper from wireguard and puts it into
linux/skbuff.h, while making it a bit more robust for general usage. In
particular, parenthesis are added around the macro argument usage, and it
now accounts for trying to iterate through an already-null skb pointer,
which will simply run the iteration zero times. This latter enhancement
means it can be used to replace both do { ... } while and while (...)
open-coded idioms.

This should take care of these three possible usages, which match all
current methods of iterations.

skb_list_walk_safe(segs, skb, next) { ... }
skb_list_walk_safe(skb, skb, next) { ... }
skb_list_walk_safe(segs, skb, segs) { ... }

Gcc appears to generate efficient code for each of these.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[ Just the skbuff.h changes for backporting - gregkh]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/skbuff.h

@@ -1480,6 +1480,11 @@ static inline void skb_mark_not_on_list(struct sk_buff *skb)
 	skb->next = NULL;
 }
 
+/* Iterate through singly-linked GSO fragments of an skb. */
+#define skb_list_walk_safe(first, skb, next)                                   \
+	for ((skb) = (first), (next) = (skb) ? (skb)->next : NULL; (skb);      \
+	     (skb) = (next), (next) = (skb) ? (skb)->next : NULL)
+
 static inline void skb_list_del_init(struct sk_buff *skb)
 {
 	__list_del_entry(&skb->list);