xfs_scrub: move disk media verification error injection

xfs_scrub: move disk media verification error injection

This isn't really disk-related since it's a knob to make the
read_verify_pool pretend that the media is defective.  Move this code
before we add a new media verify path that doesn't require the disk
abstraction.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
[djwong: split off from another hch patch, create a new commit message]
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>