powerpc/xmon: Change printk() to pr_cont()

Since some time now, printk() adds carriage return, leading to
unusable xmon output if there is no udbg backend available:

  [   54.288722] sysrq: Entering xmon
  [   54.292209] Vector: 0  at [cace3d2c]
  [   54.292274]     pc:
  [   54.292331] c0023650
  [   54.292468] : xmon+0x28/0x58
  [   54.292519]
  [   54.292574]     lr:
  [   54.292630] c0023724
  [   54.292749] : sysrq_handle_xmon+0xa4/0xfc
  [   54.292801]
  [   54.292867]     sp: cace3de8
  [   54.292931]    msr: 9032
  [   54.292999]   current = 0xc28d0000
  [   54.293072]     pid   = 377, comm = sh
  [   54.293157] Linux version 5.10.0-rc6-s3k-dev-01364-gedf13f0ccd76-dirty (root@po17688vm.idsi0.si.c-s.fr) (powerpc64-linux-gcc (GCC) 10.1.0, GNU ld (GNU Binutils) 2.34) #4211 PREEMPT Fri Dec 4 09:32:11 UTC 2020
  [   54.293287] enter ? for help
  [   54.293470] [cace3de8]
  [   54.293532] c0023724
  [   54.293654]  sysrq_handle_xmon+0xa4/0xfc
  [   54.293711]  (unreliable)
  ...
  [   54.296002]
  [   54.296159] --- Exception: c01 (System Call) at
  [   54.296217] 0fd4e784
  [   54.296303]
  [   54.296375] SP (7fca6ff0) is in userspace
  [   54.296431] mon>
  [   54.296484]  <no input ...>

Use pr_cont() instead.

Fixes: 4bcc595 ("printk: reinstate KERN_CONT for printing continuation lines")
Cc: stable@vger.kernel.org # v4.9+
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Mention that it only happens when udbg is not available]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c8a6ec704416ecd5ff2bd26213c9bc026bdd19de.1607077340.git.christophe.leroy@csgroup.eu

arch/powerpc/xmon/nonstdio.c

@@ -178,7 +178,7 @@ void xmon_printf(const char *format, ...)
 
 	if (n && rc == 0) {
 		/* No udbg hooks, fallback to printk() - dangerous */
-		printk("%s", xmon_outbuf);
+		pr_cont("%s", xmon_outbuf);
 	}
 }