ataraid module has subdisk pseudo-driver which attaches to ata diskis.
It calls ata_raid_read_metadata() function which allocate memory which shares
between all subdisks in RAID:

        if (!raidp[array]) {
            raidp[array] =
                (struct ar_softc *)malloc(sizeof(struct ar_softc), M_AR,
                                          M_NOWAIT | M_ZERO);
            if (!raidp[array]) {
                device_printf(parent, "failed to allocate metadata storage\n");
                goto adaptec_out;
            }
        }
(Similar code has all ata_raid_xxxx_read_meta functions)
Pointer to this memory stored in static local array ata_raid_arrays[]. 
This memory not freed when ataraid module unloads.

Attached patch adds freeing memory on module unload. Because ataraid(4)
module has two kernel modules - subdisk and ataraid, they both will be
unloaded when `kldunload ataraid`. And all subdisks will be detached.
So we can free shared memory between subdisks, but we should reset all
pointers before freeing, because ata_raid_subdisk_detach() method will be
called on module unload and this method can do access to this memory after
freeing.

I'm not 100% sure in correctness of this patch, so it needs review..