LU-3283 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().

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ib916635559863cd7a8aee3c88bb1d2bb2e7921b8
Reviewed-on: http://review.whamcloud.com/6275
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.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>