HTTP/2: enforce writing the sync request body buffer to file.

HTTP/2: enforce writing the sync request body buffer to file.

The sync flag of HTTP/2 request body buffer is used when the size of request
body is unknown or bigger than configured "client_body_buffer_size".  In this
case the buffer points to body data inside the global receive buffer that is
used for reading all HTTP/2 connections in the worker process.  Thus, when the
sync flag is set, the buffer must be flushed to a temporary file, otherwise
the request body data can be overwritten.

Previously, the sync buffer wasn't flushed to a temporary file if the whole
body was received in one DATA frame with the END_STREAM flag and wasn't
copied into the HTTP/2 body preread buffer.  As a result, the request body
might be corrupted (ticket #1384).

Now, setting r->request_body_in_file_only enforces writing the sync buffer
to a temporary file in all cases.