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>

include/linux/netlink.h

@@ -242,6 +242,7 @@ __nlmsg_put(struct sk_buff *skb, u32 pid, u32 seq, int type, int len, int flags)
 	nlh->nlmsg_flags = flags;
 	nlh->nlmsg_pid = pid;
 	nlh->nlmsg_seq = seq;
+	if (!__builtin_constant_p(size) || NLMSG_ALIGN(size) - size != 0)
 		memset(NLMSG_DATA(nlh) + len, 0, NLMSG_ALIGN(size) - size);
 	return nlh;
 }