storaged: fix excessive accounting when io usage has negative delta

When current io bytes are smaller than previous bytes, add 0 to delta
instead of adding current io bytes.

Bug: 64317562
Merged-In: If0c9814892ad61b790baa6395649af10b11d5b7c
Change-Id: I1ffd7ac33649ab3b8405c83b1328fa9bf49702c0
1 file changed