MAINTAINERS: update an e-mail address

I no longer have direct access to my Inktank e-mail.  I still pay
attention to rbd, so update its entry in MAINTAINERS accordingly.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Sage Weil <sage@inktank.com>

MAINTAINERS

@@ -6962,7 +6962,7 @@ F:	drivers/media/parport/*-qcam*
 RADOS BLOCK DEVICE (RBD)
 M:	Yehuda Sadeh <yehuda@inktank.com>
 M:	Sage Weil <sage@inktank.com>
-M:	Alex Elder <elder@inktank.com>
+M:	Alex Elder <elder@kernel.org>
 M:	ceph-devel@vger.kernel.org
 W:	http://ceph.com/
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/sage/ceph-client.git