LU-13180 osc: disable ext merging for rdma only pages and non-rdma

LU-13180 osc: disable ext merging for rdma only pages and non-rdma

This patch try to add logic to prevent CPU memory pages and RDMA
memory pages from merging into one RPC, codes which set OBD_BRW_RDMA_ONLY
will be added whenever RDMA only codes added later.

Change-Id: I11e8beda52cc533f17b2a40c34713f441e93d5b6
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/37567
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>