CID 1108447 Uninitialized scalar field
CID 1108333, 1108192, 1108443

* 1108333 Out-of-bounds access
* 1108192 Operands don't affect result
* 1108443 Uninitialized pointer field
CID 1108428 Uninitialized scalar field
scsi: checks for B_OK instead of SCSI_REQ_CMP in scsi_scan_bus()

* scsi_scan_lun() actually returns a status_t
CID 1108455 Structurally dead code
scsi: define SCSI_DEVICE_MAX_LUN_COUNT to set a custom max lun count.

* virtio_scsi can have 16384 luns, though we cap at 256 as our scsi_ccb
only uses uchar as a type for target_lun and target_id members.
* minor code cleanup in scsi_scan_bus().