ext3: Update MAINTAINERS for ext3 and JBD

Stephen agreed that he's unlikely to find time for working on ext3/JBD in the
near future and is not working on it for some time already. So remove him.
Added myself to JBD since after Andrew I'm probably the second most sensible
contact ;).

CC: Stephen Tweedie <sct@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>