xfs: log unmount events on console

There are times, when doing triage and forensics,
that we would like to know whether a filesystem was unmounted,
or if the plug was pulled without a clean unmount.  Log
unmounts at the same level (NOTICE) as we log mounts.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>