MFC r351706:

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.