EH method APIs

[Christoph Hellwig <hch@infradead.org>]

One think I noticed when doing the SCSI MQ work is that our EH method
signature are starting to really get into the way by passing a scsi_cmnd
as the only argument.  While we'll obviously need the command we want
to abort for eh_abort the resets aren't command specific at all and
passing the command doesn't seem too helpful in general.

There's two specific reasons why it's getting in it's way:

 - With the cmd_size field in the host template we can now allocate
   driver specific data as part of the scsi_cmnd, but we'll usually
   still need driver specific data to do the actual error handling.
   The virtio_scsi driver conversion I posted is a good example of that.
 - The scsi_reset_provider situation is getting worse: this fakes up
   a request on stack, then allocates a scsi_command which doesn't get
   fully set up and points to it and calls the eh_reset* methods on it.
   For now we can keep doing that even with blk-mq, but if we eventually
   want to remove the old code we need a way to fake up the request/cmnd
   combo.  Even until then drivers get a command that subtly different
   from normal ones from scsi_reset_provider in the old code case, and
   even more subtly different for scsi-mq.

I wonder if we should start adding new methods that pass a tmf context
soon.  I think Hannes was looking into EH methods that match the SAM
error handling concepts better anyway, so this might be a good synergy.