Right now we have one type of barrier in the block layer, and that is
used mainly for making sure that the io scheduler doesn't reorder
requests when we don't want it to.  We also need a flag to tell the io
scheduler and low level queue that this is a barrier.  So basically two
needs:

o software barrier, prevents the io scheduler from reordering
o hardware barrier, driver must prevent drive from reordering

So this patch gets rid of REQ_BARRIER and instead adds REQ_SOFTBARRIER
and REQ_HARDBARRIER.