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 <[e-mail]>
Reviewed-by: Alex Elder <[e-mail]>

drivers/block/rbd.c

···
 {
 	struct rbd_img_request *img_request;
 
-	img_request = kmem_cache_alloc(rbd_img_request_cache, GFP_ATOMIC);
+	img_request = kmem_cache_alloc(rbd_img_request_cache, GFP_NOIO);
 	if (!img_request)
 		return NULL;