rbd: allocate img_request with GFP_NOIO instead GFP_ATOMIC

Now that rbd_img_request_create() is called from work functions, no
need to use GFP_ATOMIC.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>