mm/page_alloc: Mark pagesets as __maybe_unused

Commit 9983a9d ("locking/local_lock: Make the empty local_lock_*()
function a macro.") in the -tip tree converted the local_lock_*()
functions into macros, which causes a warning with clang with
CONFIG_PREEMPT_RT=n + CONFIG_DEBUG_LOCK_ALLOC=n:

  mm/page_alloc.c:131:40: error: variable 'pagesets' is not needed and will not be emitted [-Werror,-Wunneeded-internal-declaration]
  static DEFINE_PER_CPU(struct pagesets, pagesets) = {
                                         ^
  1 error generated.

Prior to that change, clang was not able to tell that pagesets was
unused in this configuration because it does not perform cross function
analysis in the frontend. After that change, it sees that the macros
just do a typecheck on the lock member of pagesets, which is evaluated
at compile time (so the variable is technically "used"), meaning the
variable is not needed in the final assembly, as the warning states.

Mark the variable as __maybe_unused to make it clear to clang that this
is expected in this configuration so there is no more warning.

Link: ClangBuiltLinux/linux#1593
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20220215184322.440969-1-nathan@kernel.org

mm/page_alloc.c

@@ -128,7 +128,7 @@ static DEFINE_MUTEX(pcp_batch_high_lock);
 struct pagesets {
 	local_lock_t lock;
 };
-static DEFINE_PER_CPU(struct pagesets, pagesets) = {
+static DEFINE_PER_CPU(struct pagesets, pagesets) __maybe_unused = {
 	.lock = INIT_LOCAL_LOCK(lock),
 };