FileBufferingReadStream _maxRentedBufferSize is used as an exclusive bound

[NinoFloris]

FileBufferingReadStream _maxRentedBufferSize is checked as an exclusive bound staying just below 1MB.

We currently carry around our own constant that computes 1024 * 1024 -1 just to stay under that slightly unnatural max buffer rent size.

In the interest of usability it should probably just respect any size up to and including 1MB.

[analogrelay]

So you're basically saying that this code should be a <= instead of a <? Seems reasonable. Interested in sending a quick PR?

[cjaliaga]

We can do this change as part of #11569 or create a quick one here with the change to have it isolated.