scsi: handle flush errors properly

Flush commands don't transfer data and thus need to be special cased
in the I/O completion handler so that we can propagate errors to
the block layer and filesystem.

Signed-off-by: James Bottomley <JBottomley@Parallels.com>
Reported-by: Steven Haber <steven@qumulo.com>
Tested-by: Steven Haber <steven@qumulo.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Cc: stable@vger.kernel.org
Signed-off-by: Christoph Hellwig <hch@lst.de>