vinyl: fix quota watermark calculation

The code calculating the quota watermark was written long ago, when we
didn't have the common memory level and didn't use lsregion allocator
and hence it was possible to free all memory used by any range. Things
have changed drastically since then: now it's impossible to free memory
occupied by a range or even an index, because statements are shared
between indexes, so the scheduler effectively dumps all memory on
exceeding the quota. Fix the quota calculation accordingly.