MAINTAINERS: update XFS maintainer entry

I will no longer be maintaining XFS for SGI.  Ben Myers
(bpm@sgi.com) has agreed to be the primary maintainer
for XFS in my place.  I will continue to be able to push
commits to the SGI XFS tree if required.  As such I will
continue to be a designated XFS maintainer, but plan to
serve in more of a backup role.

Signed-off-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>