xfs: optimize xfs_reflink_end_cow

Instead of doing a full extent list search for each extent that is
to be deleted using xfs_bmapi_read and then doing another one inside
of xfs_bunmapi_cow use the same scheme that xfs_bumapi uses:  look
up the last extent to be deleted and then use the extent index to
walk downward until we are outside the range to be deleted.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>