x86/ldt: use "pr_info_once()" instead of open-coding it badly

Using a mutex for "print this warning only once" is so overdesigned as
to be actively offensive to my sensitive stomach.

Just use "pr_info_once()" that already does this, although in a
(harmlessly) racy manner that can in theory cause the message to be
printed twice if more than one CPU races on that "is this the first
time" test.

[ If somebody really cares about that harmless data race (which sounds
  very unlikely indeed), that person can trivially fix printk_once() by
  using a simple atomic access, preferably with an optimistic non-atomic
  test first before even bothering to treat the pointless "make sure it
  is _really_ just once" case.

  A mutex is most definitely never the right primitive to use for
  something like this. ]

Yes, this is a small and meaningless detail in a code path that hardly
matters.  But let's keep some code quality standards here, and not
accept outrageously bad code.

Link: https://lore.kernel.org/lkml/CAHk-=wgV9toS7GU3KmNpj8hCS9SeF+A0voHS8F275_mgLhL4Lw@mail.gmail.com/
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/ldt.c

@@ -559,16 +559,7 @@ static bool allow_16bit_segments(void)
 	 * is worthwhile, disallow 16-bit segments under Xen PV.
 	 */
 	if (xen_pv_domain()) {
-		static DEFINE_MUTEX(xen_warning);
-		static bool warned;
-
-		mutex_lock(&xen_warning);
-		if (!warned) {
-			pr_info("Warning: 16-bit segments do not work correctly in a Xen PV guest\n");
-			warned = true;
-		}
-		mutex_unlock(&xen_warning);
-
+		pr_info_once("Warning: 16-bit segments do not work correctly in a Xen PV guest\n");
 		return false;
 	}
 #endif