Use if ... else when printing memory attributes

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.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D29703