rdma: don't use negative index to array

Reviewed-by: Michael R. Hines <mrhines@us.ibm.com>
Signed-off-by: Isaku Yamahata <yamahata@private.email.ne.jp>
Signed-off-by: Michael R. Hines <mrhines@us.ibm.com>
Message-id: 1375584894-9917-5-git-send-email-mrhines@linux.vnet.ibm.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
