mx sstable reader: remove row_body_prev_size_label
mx sstable reader: remove row_body_prev_size_label

row_body_prev_size_label is only reached consecutively, or from
a goto not far after the label. This is changed to a while loop
starting at the label and ending at the goto.

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