repair: Avoid calling get_repair_rows_size in get_sync_boundary
repair: Avoid calling get_repair_rows_size in get_sync_boundary

Instead of calling get_repair_rows_size() which might stall with large
number of rows, return the size of the rows from read_rows_from_disk.

avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla