net: netlink: Remove the dump_cb_mutex field from struct netlink_sock

Commit 5fbf57a937f4 ("net: netlink: remove the cb_mutex "injection" from
netlink core") has removed the usage of the 'dump_cb_mutex' field from the
struct netlink_sock.

Remove the field itself now. It saves a few bytes in the structure.

Signed-off-by: Christophe JAILLET <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/netlink/af_netlink.h

···
 	struct netlink_callback	cb;
 	struct mutex		nl_cb_mutex;
 
-	struct mutex		*dump_cb_mutex;
 	void			(*netlink_rcv)(struct sk_buff *skb);
 	int			(*netlink_bind)(struct net *net, int group);
 	void			(*netlink_unbind)(struct net *net, int group);