mm/page_alloc: mark pagesets as __maybe_unused

mm/page_alloc: mark pagesets as __maybe_unused

Commit 9983a9d577db ("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: https://github.com/ClangBuiltLinux/linux/issues/1593
Link: https://lkml.kernel.org/r/20220215184322.440969-1-nathan@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>