ceph: hold extra reference to r_parent over life of request

Currently, we just assume that it will stick around by virtue of the
submitter's reference, but later patches will allow the syscall to
return early and we can't rely on that reference at that point.

While I'm not aware of any reports of it, Xiubo pointed out that this
may fix a use-after-free.  If the wait for a reply times out or is
canceled via signal, and then the reply comes in after the syscall
returns, the client can end up trying to access r_parent without a
reference.

Take an extra reference to the inode when setting r_parent and release
it when releasing the request.

Cc: stable@vger.kernel.org
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>