xfs_scrub: return bytes verified from a SCSI VERIFY command

xfs_scrub: return bytes verified from a SCSI VERIFY command

Since disk_scsi_verify and pread are interchangeably called from
disk_read_verify(), we must return the number of bytes verified (or -1)
just like what pread returns.  This doesn't matter now due to bugs in
scrub, but we're about to fix those bugs.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>