staging/lustre/osc: split different type of IO

Do not allow different type of pages at the same rpc.

Signed-off-by: Alexander Boyko <alexander_boyko@xyratex.com>
Signed-off-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Xyratex-bug-id: MRP-859
Reviewed-on: http://review.whamcloud.com/10930
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3192
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>