rados: fix 'rados df --format=json' field names

read_bytes -> read_ops
read_kb -> read_bytes (and * 1024)

Same for writes.

Remove the _kb fields entirely; they are redundant.

Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@redhat.com>
Could use a release note about the removed fields in case someone's monitoring relied on it