tcp: add missing htmldocs for skb->ll_node and sk->defer_list

Add missing entries to fix these "make htmldocs" warnings.

./include/linux/skbuff.h:953: warning: Function parameter or member 'll_node' not described in 'sk_buff'
./include/net/sock.h:540: warning: Function parameter or member 'defer_list' not described in 'sock'

Fixes: f35f8219

 ("tcp: defer skb freeing after socket lock is released")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>