xfs: consolidate sysfs ops

As a part of the series to move xfs global stats from procfs to sysfs,
this patch consolidates the sysfs ops functions and removes redundancy.

Signed-off-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>