kern/rman: update DPRINTF() macro, avoid semicolon swallowing match function

Using a variadic macro allows passing everything properly to printf().
Using the do { } while(0) construct ensures the macro acts like any
other single statement. This shows just how long some of this has
existed.

Reviewed by: imp,jhb
Pull Request: https://github.com/freebsd/freebsd-src/pull/1224