[#2823] Writing DefaultFileRegion with EpollSocketChannel may cause hang

Motivation:

In EpollSocketchannel.doWriteFileRegion(...) we need to make sure we write until sendFile(...) returns either 0 or all is written. Otherwise we may not get notified once the Channel is writable again.

This is the case as we use EPOLL_ET.

Modifications:

Always write until either sendFile returns 0 or all is written.

Result:

No more hangs when writing DefaultFileRegion can happen.

transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollSocketChannel.java

@@ -276,7 +276,7 @@ public final class EpollSocketChannel extends AbstractEpollChannel implements So
         boolean done = false;
         long flushedAmount = 0;
 
-        for (int i = config().getWriteSpinCount() - 1; i >= 0; i --) {
+        for (;;) {
             final long offset = region.transfered();
             final long localFlushedAmount = Native.sendfile(fd, region, baseOffset, offset, regionCount - offset);
             if (localFlushedAmount == 0) {