When a block goes through cow, we update the reference counts of
everything that block points to.  The internal pointers of the block
can be in just about any order, and it is likely to have clusters of
things that are close together and clusters of things that are not.

To help reduce the seeks that come with updating all of these reference
counts, sort them by byte number before actual updates are done.

Signed-off-by: Chris Mason <chris.mason@oracle.com>