netlink: avoid memset of 0 bytes sparse warning

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: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>