LU-761: Check return of ll_prep_md_op_data() using IS_ERR().

LU-761: Check return of ll_prep_md_op_data() using IS_ERR().

Change-Id: I329fa923b95ae612e50f63641d2c34b121b0f6b9
Signed-off-by: John L. Hammond <jhammond@tacc.utexas.edu>
Reviewed-on: http://review.whamcloud.com/1520
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>