Add functionality to init slot indexing.

Slot indexing is for the host to track which slots (or tags) are free
and which are used.
Signed-off-by: NJohn Garry <john.garry@huawei.com>
Reviewed-by: NArnd Bergmann <arnd@arndb.de>
Reviewed-by: NHannes Reinecke <hare@suse.de>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>