xsk: drain continuation descs after overflow in xsk_build_skb()

In the Linux kernel, the following vulnerability has been resolved: xsk: drain continuation descs after overflow in xsk_build_skb() Fix generic xmit path multi-buffer logic when packets are either too big (count of descriptors exceed MAX_SKB_FRAGS) or an invalid descriptor is included in fragmented packet. Introduce xdp_sock::drain_cont and act upon this flag - when it is set, keep on consuming descriptors from AF_XDP Tx ring and put them directly onto Cq. Previously these descriptors were silently lost and could never be reached again.