intel: Replace wraparound test logic in bufmgr_fake. Again.

I'd swapped the operands, so if we weren't in lockstep with the hardware we
said the sequence was always passed.  Additionally, a race was available that
we might have failed at recovering from.  Instead, I've replaced the logic
with new stuff that should be more robust and not rely on all the parties in
userland following the same IRQ_EMIT() == 1 protocol.  Also, in a radical
departure from past efforts, include a long comment describing the failure
modes and how we're working around them.

Thanks to haihao for catching the original issue.