mm/slub: fix bulk alloc and free stats

The SLUB sysfs stats enabled CONFIG_SLUB_STATS have two deficiencies
identified wrt bulk alloc/free operations:

- Bulk allocations from cpu freelist are not counted. Add the
  ALLOC_FASTPATH counter there.

- Bulk fastpath freeing will count a list of multiple objects with a
  single FREE_FASTPATH inc. Add a stat_add() variant to count them all.

Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>