percpu: add a test case for the specific 64-bit value addition

It might be a corner case when we add UINT_MAX as 64-bit unsigned value to
the percpu variable as it's not the same as -1 (ULONG_LONG_MAX). Add a
test case for that.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Andy Shevchenko <[e-mail]>
Cc: Borislav Petkov (AMD) <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Dennis Zhou <[e-mail]>
Cc: "H. Peter Anvin" <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Uros Bizjak <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/percpu_test.c

···
 // SPDX-License-Identifier: GPL-2.0-only
+#include <linux/limits.h>
 #include <linux/module.h>
 
 /* validate @native and @pcp counter values match @expected */
···
 	 * +ul_one/-ul_one below would replace with inc/dec instructions.
 	 */
 	volatile unsigned int ui_one = 1;
-	long l = 0;
+	unsigned long long ull = 0;
 	unsigned long ul = 0;
+	long l = 0;
 
 	pr_info("percpu test start\n");
 
···
 	__this_cpu_sub(ulong_counter, ui_one);
 	CHECK(ul, ulong_counter, -1);
 	CHECK(ul, ulong_counter, ULONG_MAX);
+
+	ul = ull = 0;
+	__this_cpu_write(ulong_counter, 0);
+
+	ul = ull += UINT_MAX;
+	__this_cpu_add(ulong_counter, ull);
+	CHECK(ul, ulong_counter, UINT_MAX);
 
 	ul = 3;
 	__this_cpu_write(ulong_counter, 3);