kernel/watchdog.c: use better pr_fmt prefix

kernel/watchdog.c: use better pr_fmt prefix

After commit 73ce0511c436 ("kernel/watchdog.c: move hardlockup
detector to separate file"), 'NMI watchdog' is inappropriate in
kernel/watchdog.c, using 'watchdog' only.

Link: http://lkml.kernel.org/r/1499928642-48983-1-git-send-email-wangkefeng.wang@huawei.com
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Babu Moger <babu.moger@oracle.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>