scsi_dh: move 'dh_state' sysfs attribute to generic code

As scsi_dh.c is now always compiled in we should be moving
the 'dh_state' attribute to the generic code.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.com>
Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>