printk: drop in_nmi check from printk_safe_flush_on_panic()

Drop the in_nmi() check from printk_safe_flush_on_panic()
and attempt to re-init (IOW unlock) locked logbuf spinlock
from panic CPU regardless of its context.

Otherwise, theoretically, we can deadlock on logbuf trying to flush
per-CPU buffers:

  a) Panic CPU is running in non-NMI context
  b) Panic CPU sends out shutdown IPI via reboot vector
  c) Panic CPU fails to stop all remote CPUs
  d) Panic CPU sends out shutdown IPI via NMI vector
     One of the CPUs that we bring down via NMI vector can hold
     logbuf spin lock (theoretically).

Link: http://lkml.kernel.org/r/20180530070350.10131-1-sergey.senozhatsky@gmail.com
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

kernel/printk/printk_safe.c

@@ -281,7 +281,7 @@ void printk_safe_flush_on_panic(void)
 	 * Make sure that we could access the main ring buffer.
 	 * Do not risk a double release when more CPUs are up.
 	 */
-	if (in_nmi() && raw_spin_is_locked(&logbuf_lock)) {
+	if (raw_spin_is_locked(&logbuf_lock)) {
 		if (num_online_cpus() > 1)
 			return;