Request body: avoid linking rb->buf to r->header_in.

Code to reuse of r->request_body->buf in upstream module assumes it's
dedicated buffer, hence after 1.3.9 (r4931) it might reuse r->header_in
if client_body_in_file_only was set, resulting in original request
corruption.  It is considered to be safer to always create a dedicated
buffer for rb->bufs to avoid such problems.