libceph: add an ONSTACK initializer for oids

An on-stack oid in ceph_ioctl_get_dataloc() is not initialized,
resulting in a WARN and a NULL pointer dereference later on.  We will
have more of these on-stack in the future, so fix it with a convenience
macro.

Fixes: d30291b985d1 ("libceph: variable-sized ceph_object_id")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/fs/ceph/ioctl.c b/fs/ceph/ioctl.c
index be6b165..0946f2d 100644
--- a/fs/ceph/ioctl.c
+++ b/fs/ceph/ioctl.c
@@ -183,7 +183,7 @@
 	struct ceph_osd_client *osdc =
 		&ceph_sb_to_client(inode->i_sb)->client->osdc;
 	struct ceph_object_locator oloc;
-	struct ceph_object_id oid;
+	CEPH_DEFINE_OID_ONSTACK(oid);
 	u64 len = 1, olen;
 	u64 tmp;
 	struct ceph_pg pgid;