MFC: r264888
The PR reported that the old NFS server did not set uio_td == NULL
for the VOP_READ() call. This patch fixes both the old and new
server for this case.
MFC r264313:
Do not increment bio_data in case of BIO_DELETE.

This fixes KASSERT() panic in g_io_request().
MFC r265054:
Reduce number of opens by REOM 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.

Sponsored by:	iXsystems, Inc.