xfs: per-filesystem stats in sysfs

This patch implements per-filesystem stats objects in sysfs. It
depends on the application of the previous patch series that
develops the infrastructure to support both xfs global stats and
xfs per-fs stats in sysfs.

Stats objects are instantiated when an xfs filesystem is mounted
and deleted on unmount. With this patch, the stats directory is
created and populated with the familiar stats and stats_clear files.
Example:
        /sys/fs/xfs/sda9/stats/stats
        /sys/fs/xfs/sda9/stats/stats_clear

With this patch, the individual counts within the new per-fs
stats file(s) remain at zero. Functions that use the the macros
to increment, decrement, and add-to the per-fs stats counts will
be covered in a separate new patch to follow this one. Note that
the counts within the global stats file (/sys/fs/xfs/stats/stats)
advance normally and can be cleared as it was prior to this patch.

[dchinner: move setup/teardown to xfs_fs_{fill|put}_super() so
it is down before/after any path that uses the per-mount 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>