btrfs: get rid of sector_t and use u64 offset in submit_extent_page

The use of sector_t in the callchain of submit_extent_page is not
necessary.  Switch to u64 and rename the variable and use byte units
instead of 512b, ie.  dropping the >> 9 shifts and avoiding the
con(tro)versions of sector_t.

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