ceph: don't leave ino field in ceph_mds_request_head uninitialized

We currently just pass junk in this field unless we're retransmitting a
create, but in later patches, we'll need a mechanism to pass a delegated
inode number on an initial create request. Prepare for this by ensuring
this field is zeroed out.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>