Reduce number of opens by GEOM RAID during provider taste.

Instead opening/closing provider by each of metadata classes, do it only
once in core code.  Since for SCSI disks open/close means sending some
SCSI commands to the device, this change reduces taste time.

Ticket:	#4870

(cherry picked from commit b384f8775fb9dac291a6c48827c9f7c4bd2bc455)