replace DIV_ROUND with DIV_ROUND_CLOSEST

The Linux-compatible macro DIV_ROUND_CLOSEST is a bit more flexible
and safer than DIV_ROUND.

For example,
  foo = DIV_ROUND_CLOSEST(x, y++)
works expectedly, but
  foo = DIV_ROUND(x, y++)
does not. (y is incremented twice.)

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>