Apply a torniquet to the problem of the drive unexpectedly disconnecting

during a data phase.  Before, we would try to recover the autosense, but
the DMA engine would still be active with interrupted transfer, and we'd
quickly spiral out of control and cause massive data corruption.  For now,
just reset the chip and cancel everything.  The better solution is to
cancel the DMA operation, but there is no clear way to do that right now.
The data corruption problem is severe enough to warrant this fix in the
interim.  Thanks to Kris Kenneway to sacrificing countless filesystems to
this bug.

MFC After: 3 days

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