internet: Read correct bits when deserializing IPv6 flow label

On line 153, m_flowLabel is OR-ed into the LSBs of vTcFl, which is then written with WriteHtonU32 into the buffer. On line 169, vTcFl is read out using ReadNtohU32, so the byte order should be the same as it was on line 153. However, currently m_flowLabel is being assigned the value of vTcFl masked by 0xfff00000.

This commit changes line 177 to read the correct set of 20 bits to recover the original flow label.

One question: SetFlowLabel (and the other setters) don't check that the value is in range, and Serialize doesn't mask them. Should this be fixed? Right now, a large flow label would clobber the traffic class and version fields, since these are all ORed together into vTcFl.