x86/percpu: fix clang warning when dealing with unsigned types

Patch series "percpu: Add a test case and fix for clang", v2.

Add a test case to percpu to check a corner case with the specific 64-bit
unsigned value. This test case shows why the first patch is done in the
way it's done.

The before and after has been tested with binary comparison of the
percpu_test module and runnig it on the real Intel system.


This patch (of 2):

When percpu_add_op() is used with an unsigned argument, it prevents kernel
builds with clang, `make W=1` and CONFIG_WERROR=y:

net/ipv4/tcp_output.c:187:3: error: result of comparison of constant -1 with expression of type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
187 | NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
188 | tp->compressed_ack);
| ~~~~~~~~~~~~~~~~~~~
...
arch/x86/include/asm/percpu.h:238:31: note: expanded from macro 'percpu_add_op'
238 | ((val) == 1 || (val) == -1)) ? \
| ~~~~~ ^ ~~

Fix this by casting -1 to the type of the parameter and then compare.

Link: https://lkml.kernel.org/r/20241016182635.1156168-1-andriy.shevchenko@linux.intel.com
Link: https://lkml.kernel.org/r/20241016182635.1156168-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Christoph Lameter <cl@linux.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>