To avoid having to repeat the entire definition of an attribute
(including the value) use the Attr object from the original set.
In fact this is already the documented expectation.

Fixes: be5bea1c ("net: add basic C code generators for Netlink")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>