common: Reformat some comments in iobuf.c

Comments with stars on the left side are easier to read. Also a
long comment in the form

if (...)
   /* This is a comment
    * for the next block */
  {
     ...
  }

makes it hard to see the start of a block or hides a forgotten block.
In contrast to GNU common use we don't want this but put the comment
either into the block or with adjusted wording above the condition.