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: d30291b9

 ("libceph: variable-sized ceph_object_id")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>