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>

include/net/netlink.h

@@ -1190,6 +1190,16 @@ static inline struct in6_addr nla_get_in6_addr(const struct nlattr *nla)
 	return tmp;
 }
 
+/**
+ * nla_memdup - duplicate attribute memory (kmemdup)
+ * @src: netlink attribute to duplicate from
+ * @gfp: GFP mask
+ */
+static inline void *nla_memdup(const struct nlattr *src, gfp_t gfp)
+{
+	return kmemdup(nla_data(src), nla_len(src), gfp);
+}
+
 /**
  * nla_nest_start - Start a new level of nested attributes
  * @skb: socket buffer to add attributes to