staging/lustre/llite: check ll_prep_md_op_data() using IS_ERR()

In ll_file_ioctl() and ll_swap_layouts() check the result of
ll_prep_md_op_data() using IS_ERR().

Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3283
Lustre-change: http://review.whamcloud.com/6275


Signed-off-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>