previous commit causes gcc to perform an unaligned access to the tcphdr

previous commit causes gcc to perform an unaligned access to the tcphdr
(at least on sparc64) since it accesses the bitfield using an "int sized"
instructions, rather than the minimally sized byte instruction.  This is
permitted by the language laywers who probably prefer we change the tcphdr
in every packet.  It is not clear how to convince gcc to avoid this behaviour,
and a week of futzing hasn't found fast path solutions yet.  In the meantime
the tree may not be broken.