Previously it just limited the returned write space, irrespective of how much
had already been written. The buffer could still be filled up by doing multiple
writes. Now the size is limited by adjusting the read pointer by the real vs
limited difference when calculating the writable space.
Previously, all but one of the specified size could be written (so for a size
of n, only n-1 was guaranteed writable). All users pretty much compensated for
this, but it makes more sense to fix it at the source.
