mm: thp: Fix build warning when CONFIG_SYSFS is disabled

huge_anon_orders_lock is used only to serialize sysfs writers. So move
its definition so that it is within the CONFIG_SYSFS ifdefery to
suppress "defined but not used" warning when sysfs is disabled.

Fixes: fe2c9313a8c0 ("mm: thp: Introduce multi-size THP sysfs interface")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202312111916.YbsHxKPq-lkp@intel.com/


Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>