MAINTAINERS: add missing files to slab section

The failslab implementation implements fault injection for slab allocations
so seems best suited to the slab section.

The mempool implementation uses slab in the backend, so that seems best
suited here also.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>