libnetlink: add attribute access inline functions

Based on idea in libmnl, add attribute access functions instead
of explicitly exposing casts. Also handle possible alignment issues
of u64.

include/libnetlink.h

@@ -70,6 +70,25 @@ extern int __parse_rtattr_nested_compat(struct rtattr *tb[], int max, struct rta
 	({ data = RTA_PAYLOAD(rta) >= len ? RTA_DATA(rta) : NULL;	\
 		__parse_rtattr_nested_compat(tb, max, rta, len); })
 
+static inline __u16 rta_getattr_u16(const struct rtattr *rta)
+{
+	return *(__u16 *)RTA_DATA(rta);
+}
+static inline __u16 rta_getattr_u32(const struct rtattr *rta)
+{
+	return *(__u32 *)RTA_DATA(rta);
+}
+static inline __u64 rta_getattr_u64(const struct rtattr *rta)
+{
+	__u64 tmp;
+	memcpy(&tmp, RTA_DATA(rta), sizeof(__u64));
+	return tmp;
+}
+static inline const char *rta_getattr_str(const struct rtattr *rta)
+{
+	return RTA_DATA(rta);
+}
+
 extern int rtnl_listen(struct rtnl_handle *, rtnl_filter_t handler,
 		       void *jarg);
 extern int rtnl_from_file(FILE *, rtnl_filter_t handler,