Fix data corruption in FileRegion transfer with epoll transport

Related issue: #2764

Motivation:

EpollSocketChannel.writeFileRegion() does not handle the case where the
position of a FileRegion is non-zero properly.

Modifications:

- Improve SocketFileRegionTest so that it tests the cases where the file
  transfer begins from the middle of the file
- Add another jlong parameter named 'base_off' so that we can take the
  position of a FileRegion into account

Result:

Improved test passes. Corruption is gone.