libceph: evaluate osd_req_op_data() arguments only once

This patch changes the osd_req_op_data() macro to not evaluate
arguments more than once in order to follow the kernel coding style.

Signed-off-by: Ioana Ciornei <ciorneiioana@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>
[idryomov@gmail.com: changelog, formatting]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>