fix LRSC issue (RocketChip issue #86)

It was possible that the result of a store-conditional could get lost if it
did not depend on the result of the corresponding load-reserved.

This was because the MSHR does not update the client state based on the
secondary requests. So the LR would acquire the line in clientExcusiveClean,
but then we would fail to update the metadata array to change the state
to clientExclusiveDirty.

The solution is to track whether a secondary acquire would cause the
line to be dirty. If so, use M_XWR instead of the primary command to
generate the update coherence state.