Fix smart-combine behavior of IOBuf::create
Fix smart-combine behavior of IOBuf::create

Summary: D27353914 (https://github.com/facebook/folly/commit/59b71bbba1f9d5e091cf9fcb76f3a47a809e133d) claimed to combine the buf and info allocations when doing so would not increase the allocation size, but does not appear to have added the combined path. Add it.

Reviewed By: Gownta

Differential Revision: D37352069

fbshipit-source-id: b9727e05e33c6f4b36212944abfba0f48cd0a8da

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly