mm: huge_mm: fix undefined reference to `mthp_stats' for CONFIG_SYSFS=n

if CONFIG_SYSFS is not enabled in config, we get the below error,

All errors (new ones prefixed by >>):

   s390-linux-ld: mm/memory.o: in function `count_mthp_stat':
>> include/linux/huge_mm.h:285:(.text+0x191c): undefined reference to `mthp_stats'
   s390-linux-ld: mm/huge_memory.o:(.rodata+0x10): undefined reference to `mthp_stats'

vim +285 include/linux/huge_mm.h

   279
   280  static inline void count_mthp_stat(int order, enum mthp_stat_item item)
   281  {
   282          if (order <= 0 || order > PMD_ORDER)
   283                  return;
   284
 > 285          this_cpu_inc(mthp_stats.stats[order][item]);
   286  }
   287

Link: https://lkml.kernel.org/r/20240523210045.40444-1-21cnbao@gmail.com
Fixes: ec33687 ("mm: add per-order mTHP anon_fault_alloc and anon_fault_fallback counters")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202405231728.tCAogiSI-lkp@intel.com/
Signed-off-by: Barry Song <v-songbaohua@oppo.com>
Tested-by: Yujie Liu <yujie.liu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Barry Song

include/linux/huge_mm.h

@@ -278,6 +278,7 @@ struct mthp_stat {
 	unsigned long stats[ilog2(MAX_PTRS_PER_PTE) + 1][__MTHP_STAT_COUNT];
 };
 
+#ifdef CONFIG_SYSFS
 DECLARE_PER_CPU(struct mthp_stat, mthp_stats);
 
 static inline void count_mthp_stat(int order, enum mthp_stat_item item)
@@ -287,6 +288,11 @@ static inline void count_mthp_stat(int order, enum mthp_stat_item item)
 
 	this_cpu_inc(mthp_stats.stats[order][item]);
 }
+#else
+static inline void count_mthp_stat(int order, enum mthp_stat_item item)
+{
+}
+#endif
 
 #define transparent_hugepage_use_zero_page()				\
 	(transparent_hugepage_flags &					\