f2fs: remove the unused stat_lock

struct f2fs_stat_info {

	struct list_head stat_list;

	struct f2fs_sb_info *sbi;

	struct mutex stat_lock;

	int all_area_segs, sit_area_segs, nat_area_segs, ssa_area_segs;

	int main_area_segs, main_area_sections, main_area_zones;

	int hit_ext, total_ext;