ocfs2: Move /sys/o2cb to /sys/fs/o2cb

/sys/fs is where we really want file system specific sysfs objects.

Ocfs2-tools has been updated to look in /sys/fs/o2cb. We can maintain
backwards compatibility with old ocfs2-tools by using a sysfs symlink. After
some time (2 years), the symlink can be safely removed. This patch also adds
documentation to make it easier for people to figure out what /sys/fs/o2cb
is used for.

Signed-off-by: Mark Fasheh <mfasheh@suse.com>