slub: Update frozen slabs documentations in the source

The current updated scheme (which this series implemented) is:
 - node partial slabs: PG_Workingset && !frozen
 - cpu partial slabs: !PG_Workingset && !frozen
 - cpu slabs: !PG_Workingset && frozen
 - full slabs: !PG_Workingset && !frozen

The most important change is that "frozen" bit is not set for the
cpu partial slabs anymore, __slab_free() will grab node list_lock
then check by !PG_Workingset that it's not on a node partial list.

And the "frozen" bit is still kept for the cpu slabs for performance,
since we don't need to grab node list_lock to check whether the
PG_Workingset is set or not if the "frozen" bit is set in __slab_free().

Update related documentations and comments in the source.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Tested-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Acked-by: Christoph Lameter (Ampere) <cl@linux.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>