This patch adds per-super attributes to sysfs.

It doesn't publish any attributes yet, but does the proper lifetime
handling as well as the basic infrastructure to add new attributes.
Signed-off-by: NJeff Mahoney <jeffm@suse.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>
Signed-off-by: NChris Mason <clm@fb.com>