scsi: core: scsi_io_completion convert BUGs to WARNs

The scsi_io_completion function contains three BUG() and BUG_ON() calls.
Replace them with WARN variants.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>