Use if ... else when printing memory attributes

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

(cherry picked from commit 15221c552b3cabcbf26613246e855010b176805a)