rbd: use GFP_NOIO for parent stat and data requests

rbd_img_obj_exists_submit() and rbd_img_obj_parent_read_full() are on
the writeback path for cloned images -- we attempt a stat on the parent
object to see if it exists and potentially read it in to call copyup.
GFP_NOIO should be used instead of GFP_KERNEL here.

Cc: [e-mail]
Link: http://tracker.ceph.com/issues/22014
Signed-off-by: Ilya Dryomov <[e-mail]>
Reviewed-by: David Disseldorp <[e-mail]>

drivers/block/rbd.c

···
 	 * from the parent.
 	 */
 	page_count = (u32)calc_pages_for(0, length);
-	pages = ceph_alloc_page_vector(page_count, GFP_KERNEL);
+	pages = ceph_alloc_page_vector(page_count, GFP_NOIO);
 	if (IS_ERR(pages)) {
 		result = PTR_ERR(pages);
 		pages = NULL;
···
 	 */
 	size = sizeof (__le64) + sizeof (__le32) + sizeof (__le32);
 	page_count = (u32)calc_pages_for(0, size);
-	pages = ceph_alloc_page_vector(page_count, GFP_KERNEL);
+	pages = ceph_alloc_page_vector(page_count, GFP_NOIO);
 	if (IS_ERR(pages)) {
 		ret = PTR_ERR(pages);
 		goto fail_stat_request;