skmsg: Add function doc for skb->_sk_redir

This should fix the following warning:

include/linux/skbuff.h:932: warning: Function parameter or member
'_sk_redir' not described in 'sk_buff'
Reported-by: NLorenz Bauer <lmb@cloudflare.com>
Signed-off-by: NCong Wang <cong.wang@bytedance.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NJohn Fastabend <john.fastabend@gmail.com>
Acked-by: NLorenz Bauer <lmb@cloudflare.com>
Link: https://lore.kernel.org/bpf/20210301184805.8174-1-xiyou.wangcong@gmail.com

include/linux/skbuff.h

@@ -656,6 +656,7 @@ typedef unsigned char *sk_buff_data_t;
  *	@protocol: Packet protocol from driver
  *	@destructor: Destruct function
  *	@tcp_tsorted_anchor: list structure for TCP (tp->tsorted_sent_queue)
+ *	@_sk_redir: socket redirection information for skmsg
  *	@_nfct: Associated connection, if any (with nfctinfo bits)
  *	@nf_bridge: Saved data about a bridged frame - see br_netfilter.c
  *	@skb_iif: ifindex of device we arrived on