netlink: Add nla_memdup() to wrap kmemdup() use on nlattr

Wrap several common instances of:
	kmemdup(nla_data(attr), nla_len(attr), GFP_KERNEL);

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>