cciss: Fix usage_count check in rebuild_lun_table when triggered via sysfs.

When rebuild_lun_table is reached via sysfs, the usage count that
is checked prior to messing with c0d0 has different constraints
(must be zero) than if rebuild_lun_table is reached via ioctl
(must be one.)  Fix rebuild_lun_table to take that into account.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>