Proxy: unbusy stale HTTP/2 request body buffers on next upstream.

Similarly to previous changes, if a buffered request body wasn't
fully sent, a partially sent buffer was sitting in the busy chain
and not recycled when switching to the next upstream server.
This resulted in a stalled connection because ngx_output_chain()
could not get a free buffer to read the next portion of request body.

src/http/modules/ngx_http_proxy_v2_module.c

@@ -983,6 +983,10 @@ ngx_http_proxy_v2_body_output_filter(void *data, ngx_chain_t *in)
 
             ngx_free_chain(r->pool, ln);
         }
+
+        for (ln = ctx->busy; ln; ln = ln->next) {
+            ln->buf->pos = ln->buf->last;
+        }
     }
 
     if (in) {