LinuxKPI: add no_printk

Add a version of no_printk(), which seems to be there to have format
string checking while never calling the printk. It seems a very weird
thing and it needs a return code and for some reason my initial
while (0) { } version hadn't worked while porting over new code but
could have been further downstream format string problems.
if (0) seems to do the job though I would have expected that to more
likely simply get optimised out without any futher format checking.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D46463