btrfs: scrub: get rid of sector_t

The use of sector_t is not necessry, it's just for a warning.  Switch to
u64 and rename the variable and use byte units instead of 512b, ie.
dropping the >> 9 shifts.  The messages are adjusted as well.

Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>