printk: revert ktime_get() timestamps

revert 19ef9309273d26cb005cb23e6a370353dca91099.

Kevin Winchester reported a lockup during X startup an bisected
it to this commit.

Reported-by: Kevin Winchester <kjwinchester@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/printk.c

@@ -702,9 +702,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 					loglev_char = default_message_loglevel
 						+ '0';
 				}
-				t = 0;
-				if (system_state != SYSTEM_BOOTING)
-					t = ktime_to_ns(ktime_get());
+				t = cpu_clock(printk_cpu);
 				nanosec_rem = do_div(t, 1000000000);
 				tlen = sprintf(tbuf,
 						"<%c>[%5lu.%06lu] ",