Return the maximum available space from IOBufQueue::preallocate

Return the maximum available space from IOBufQueue::preallocate

Summary: IOBufQueue::preallocate currently takes a min and max size, and will return to the caller a buffer with N writable bytes with min <= N <= max.  This is a bit wasteful, since there may be more than max bytes available at the end if the queue with no extra cost.  Now preallocate will return the full amount of contigious space to the caller, even if it is larger than the requested max.

Test Plan: folly and proxygen unit tests

Reviewed By: simpkins@fb.com

FB internal diff: D633912