Do not reread SCSI disk VPD pages on every device open.

Instead of rereading VPD pages on every device open, do it only on initial
device probe, and in cases when device reported via UNIT ATTENTIONs that
something has changed.  Capacity is still rereaded on every open because
it is more critical for operation and more probable to change in run time.

On my tests with Intel 530 SSDs on mps(4) HBA this change reduces time
GEOM needs to retaste the device (that includes few open/close cycles)
from ~150ms to ~30ms.

MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.