rbd: use GFP_NOIO for parent stat and data requests

rbd: use GFP_NOIO for parent stat and data requests

commit 1e37f2f84680fa7f8394fd444b6928e334495ccc upstream.

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.

Link: http://tracker.ceph.com/issues/22014
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>