correct inverted subtraction in quota check

From a user reported bug: we were doing the subtraction backwards
here, causing int overflow, causing all kinds of invalid. Instead
actually subtract the smaller number from the bigger number.

Change-Id: I7c099df5e31aa25b8be65c570e2e0b8a3e93c277
Closes-Bug: #1363324
Merge "correct inverted subtraction in quota check"