scsi: handle flush errors properly

commit 89fb4cd1f717a871ef79fa7debbe840e3225cd54 upstream.

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.

Change-Id: I6a1c055e728e1810bd778951b613db021c061675