libbpf: Add support for canceling cached_cons advance

Add a new function for returning descriptors the user received
after an xsk_ring_cons__peek call. After the application has
gotten a number of descriptors from a ring, it might not be able
to or want to process them all for various reasons. Therefore,
it would be useful to have an interface for returning or
cancelling a number of them so that they are returned to the ring.

This patch adds a new function called xsk_ring_cons__cancel that
performs this operation on nb descriptors counted from the end of
the batch of descriptors that was received through the peek call.

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
[ Magnus Karlsson: rewrote changelog ]
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/1606202474-8119-1-git-send-email-lirongqing@baidu.com

tools/lib/bpf/xsk.h

@@ -153,6 +153,12 @@ static inline size_t xsk_ring_cons__peek(struct xsk_ring_cons *cons,
 	return entries;
 }
 
+static inline void xsk_ring_cons__cancel(struct xsk_ring_cons *cons,
+					 size_t nb)
+{
+	cons->cached_cons -= nb;
+}
+
 static inline void xsk_ring_cons__release(struct xsk_ring_cons *cons, size_t nb)
 {
 	/* Make sure data has been read before indicating we are done