x86/ldt: Make LDT pgtable free conditional

Andy prefers to be paranoid about the pagetable free in the error path of
write_ldt(). Make it conditional and warn whenever the installment of a
secondary LDT fails.
Requested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/ldt.c

@@ -426,7 +426,8 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
 		 * already installed then the PTE page is already
 		 * populated. Mop up a half populated page table.
 		 */
-		free_ldt_pgtables(mm);
+		if (!WARN_ON_ONCE(old_ldt))
+			free_ldt_pgtables(mm);
 		free_ldt_struct(new_ldt);
 		goto out_unlock;
 	}