FFI :buffer_out params incorrectly handled as pinned arrays by jffi

When a buffer is passed to a function either as a :buffer_out parameter, or the buffer was allocated via Buffer.new_out or Struct.new_out, then the backing array will be pinned by the JVM, instead of allocating temporary native memory.

Pinning arrays turns off garbage collection until all arrays are unpinned, so a :buffer_out param on a blocking function (e.g. read()) could block GC across all threads.

A patch to work around the problem in jruby will be forthcoming.