scsi: make sure that scsi_init_shared_tag_map() doesn't overwrite existing map

Right now callers have to check whether scsi_host->bqt is already
set up, it's much cleaner to just have scsi_init_shared_tag_map()
does this check on its own.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>