MFC r351706:
In nvme_completion_poll, add a sanity check to make sure that we complete the polling within a second. Panic if we don't. All the commands that use this interface should typically complete within a few tens to hundreds of microseconds. Panic rather than return ETIMEDOUT because if the command somehow does later complete, it will randomly corrupt memory. Also, it helps to get a traceback from where the unexpected failure happens, rather than an infinite loop.