scrub: don't allocate disk for ioctl-based media verify

scrub: don't allocate disk for ioctl-based media verify

When the kernel provides the data verification ioctl there is no point
in allocating struct disk and thus opening the underlying devices.
Refactor the code so that we don't have to do that, with the added
benefit of keeping the read verification self-contained in
read_verify.c for the case where the kernel provides the ioctl.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
[djwong: break up patch]
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>