[SCSI] st: remove st_mutex

The st_mutex was created when the BKL was removed, and
prevents simultaneous st_open calls. It is better to
protect just the necessary data.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Lee Duncan <lduncan@suse.com>
Acked-by: Kai Mäkisara <Kai.Makisara@kolumbus.fi>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>