mm/slab: allow specifying free pointer offset when using constructor

mm/slab: allow specifying free pointer offset when using constructor

When a slab cache has a constructor, the free pointer is placed after the
object because certain fields must not be overwritten even after the
object is freed.

However, some fields that the constructor does not initialize can safely
be overwritten after free. Allow specifying the free pointer offset within
the object, reducing the overall object size when some fields can be reused
for the free pointer.

Adjust the document accordingly.

Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20260113061845.159790-3-harry.yoo@oracle.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>