Currently we have a single function slab_free() handling both single
object freeing and bulk freeing with necessary hooks, the latter case
requiring slab_free_freelist_hook(). It should be however better to
distinguish the two use cases for the following reasons:

- code simpler to follow for the single object case

- better code generation - although inlining should eliminate the
  slab_free_freelist_hook() for single object freeing in case no
  debugging options are enabled, it seems it's not perfect. When e.g.
  KASAN is enabled, we're imposing additional unnecessary overhead for
  single object freeing.

- preparation to add percpu array caches in near future

Therefore, simplify slab_free() for the single object case by dropping
unnecessary parameters and calling only slab_free_hook() instead of
slab_free_freelist_hook(). Rename the bulk variant to slab_free_bulk()
and adjust callers accordingly.

While at it, flip (and document) slab_free_hook() return value so that
it returns true when the freeing can proceed, which matches the logic of
slab_free_freelist_hook() and is not confusingly the opposite.

Additionally we can simplify a bit by changing the tail parameter of
do_slab_free() when freeing a single object - instead of NULL we can set
it equal to head.

bloat-o-meter shows small code reduction with a .config that has KASAN
etc disabled:

add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-118 (-118)
Function                                     old     new   delta
kmem_cache_alloc_bulk                       1203    1196      -7
kmem_cache_free                              861     835     -26
__kmem_cache_free                            741     704     -37
kmem_cache_free_bulk                         911     863     -48
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>