CVE-2026-99057: xen/netfront: don't leak the skb when xennet_fill_frags() fails

In the Linux kernel, the following vulnerability has been resolved:

xen/netfront: don't leak the skb when xennetfillfrags() fails

When a response chain has more slots than fit in the skb's frags, xennetfillfrags() returns an error and xennetpoll() jumps to its error path. That path moves what's left on tmpq to errq to be freed, but the skb being filled was already dequeued from tmpq, so it's never freed. Each chain that overflows leaks the skb and the pages attached to it as frags, and the backend decides how many slots it sends.

Put the skb back on tmpq before taking the error path, like the xennetsetskbgso() failure just above it does.