Make returnRequest actually return the read buffer to readPool

Due to a typo, returnRequest never actually returned the read buffer
to readPool.

Also, we want to put the complete buffer back into the pool - this
caused errors later on.

loopback "tar xzf ../linux-3.0.tar.gz" timings:

before: 35 seconds
after:  10 seconds

fuse/request.go

@@ -126,7 +126,7 @@ func (r *request) setInput(input []byte) bool {
 		return false
 	}
 	r.inputBuf = input
-	r.bufferPoolInputBuf = input
+	r.bufferPoolInputBuf = input[:cap(input)]
 
 	return true
 }

fuse/server.go

@@ -283,7 +283,7 @@ func (ms *Server) returnRequest(req *request) {
 
 	req.clear()
 	ms.reqMu.Lock()
-	if req.bufferPoolOutputBuf != nil {
+	if req.bufferPoolInputBuf != nil {
 		ms.readPool.Put(req.bufferPoolInputBuf)
 		req.bufferPoolInputBuf = nil
 		ms.outstandingReadBufs--