block: rbd: update sysfs interface

The existing sysfs interface has been updated to be in the same format
as described in Documentation/ABI/README. This will be useful for
scripting and tracking changes in the ABI. Attributes have been grouped
by functionality and/or the date on which they were added.

There are a couple of more changes:
- The attributes have been annotated with file permissions RO/RW/WO.
- Added description of the bus attribute supported_features

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>