Use if ... else when printing memory attributes

[andrew]

Summary

In vmstat there is a switch statement that converts these attributes to
a string. As some values can be duplicate we have to hide these from
userspace.

Replace this switch statement with an if ... else macro that lets us
repeat values without a compiler error.

[jhb]

I suppose if this had been an enum there would have been value in the switch to avoid missing new values in the future, but it's not an enum.