lib/atomic64_test.c: convert printk(KERN_INFO to pr_info

Convert printk to current pr_foo() logging functions.

Also add pr_fmt based on KBUILD_MODNAME to avoid repeating prefix.  Prefix
is now "atomic64_test: "

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Luca Barbieri <luca@luca-barbieri.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>