lib/bug.c: convert printk to pr_foo()

- Coalesce formats

- "WARNING:" prefix unchanged to keep bug format.

- printk(KERN_DEFAULT not converted.

- define pr_fmt without prefix to avoid any default prefix update
  (suggested by Joe Perches).

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>