mgr/diskprediction_local: Fix array size error

mgr/diskprediction_local: Fix array size error

The disk predictor assumes that it has at least 6 days worth of data to work
with, and the code ensures that `health_data` contains at least 6 elements.

However, it then gets fitlered down into a `predict_datas` array that can
contain less than 6 elements in some cases.

This commit ensures that `predict_datas` contains 6 elements or more before
passing it on to the disk predictor.

Fixes: https://tracker.ceph.com/issues/46549
Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>