mem: clean up write buffer check in Cache::handleSnoop()

mem: clean up write buffer check in Cache::handleSnoop()

The 'if (writebacks.size)' check was redundant, because
writeBuffer.findMatches() would return false if the
writebacks list was empty.

Also renamed 'mshr' to 'wb_entry' in this context since
we are pointing at a writebuffer entry and not an MSHR
(even though it's the same C++ class).