MAINTAINERS: Pass the torch of SRP submaintainership

Today was my last day at ORNL, and my future endeavors will leave even
less time to maintain the SRP initiator.

My thanks especially go to Bart, for keeping the pressure to improve
alive, and for driving so many of those improvements.

[ Add Bart as new submaintainer.  - Roland ]

Signed-off-by: Roland Dreier <roland@purestorage.com>