improve the kmem_cache_alloc_bulk API v2

Hi all,

kmem_cache_alloc_bulk has a very unintuitive and undocumented return
value convention.  Fix that and add documentation.

Changes since v1:
 - fix compile in drm and test_meminit
 - document that 0-sized allocations fail

Diffstat:
 drivers/gpu/drm/msm/msm_iommu.c       |    6 +--
 drivers/gpu/drm/panthor/panthor_mmu.c |   13 +++----
 include/linux/slab.h                  |    6 ++-
 io_uring/io_uring.c                   |   23 +++++--------
 lib/test_meminit.c                    |   23 ++++++-------
 mm/kasan/kasan_test_c.c               |    5 +-
 mm/kfence/kfence_test.c               |    9 ++---
 mm/slub.c                             |   59 +++++++++++++++++++---------------
 net/bpf/test_run.c                    |    7 +---
 net/core/skbuff.c                     |   24 +++++++------
 tools/include/linux/slab.h            |    2 -
 tools/testing/shared/linux.c          |   19 ++++------
 12 files changed, 97 insertions(+), 99 deletions(-)