x86/kasan: Add message about KASAN being initialized

Print informational message to tell user that kernel
runs with KASAN enabled.

Add a "kasan: " prefix to all messages in kasan_init_64.c.

Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Cc: Alexander Popov <alpopov@ptsecurity.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <adech.fo@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1435828178-10975-6-git-send-email-a.ryabinin@samsung.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>