dm io: fix the sync/async helper

Without this patch they are actually the same thing since the changes
against the rw field does not get passed to the under level routines.

It seems like a mistake during rebase to RHEL 2.6.32-220.

Signed-off-by: Zhu Yanhai <gaoyang.zyh@taobao.com>
Merge pull request #73 from jesuszhu/dm-fix

dm io: fix the sync/async helper