mm/slab: only allow SLAB_OBJ_EXT_IN_OBJ for unmergeable caches

mm/slab: only allow SLAB_OBJ_EXT_IN_OBJ for unmergeable caches

While SLAB_OBJ_EXT_IN_OBJ allows to reduce memory overhead to account
slab objects, it prevents slab merging because merging can change
the metadata layout.

As pointed out Vlastimil Babka, disabling merging solely for this memory
optimization may not be a net win, because disabling slab merging tends
to increase overall memory usage.

Restrict SLAB_OBJ_EXT_IN_OBJ to caches that are already unmergeable for
other reasons (e.g., those with constructors or SLAB_TYPESAFE_BY_RCU).

Suggested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
Link: https://patch.msgid.link/20260127103151.21883-3-harry.yoo@oracle.com
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>