netdev-linux: Avoid "cleverness" in swap_uint64().

netdev-linux: Avoid "cleverness" in swap_uint64().

Obviously correct code is easier on everyone.  As the C FAQ says:

20.15c:	How can I swap two values without using a temporary?

A:	The standard hoary old assembly language programmer's trick is:

		a ^= b;
		b ^= a;
		a ^= b;

	But this sort of code has little place in modern, HLL
	programming.  Temporary variables are essentially free,
	and the idiomatic code using three assignments, namely

		int t = a;
		a = b;
		b = t;

	is not only clearer to the human reader, it is more likely to be
	recognized by the compiler and turned into the most-efficient
	code (e.g. using a swap instruction, if available).  The latter
	code is obviously also amenable to use with pointers and
	floating-point values, unlike the XOR trick.  See also questions
	3.3b and 10.3.