xfs: create symlink proc/fs/xfs/stat to sys/fs/xfs/stats

As a part of the work to move xfs global stats from procfs to sysfs,
this patch creates the symlink from proc/fs/xfs/stat to sys/fs/xfs/stats.

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