A netlink attribute padding of zero triggers this sparse warning:

include/linux/netlink.h:245:8: warning: memset with byte count of 0

Avoid the memset when the size parameter is constant and requires no padding.
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>